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Cornell University Library 
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Cornell University 

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A TEXT-BOOK OF \:!>,,X^ 








M. A., M. D., L. R. C.T. (eNG.) 








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Cot^RIGflT, 18 


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The comparative method, the introduction of the teach- 
ings of embryology and of the welding principles of evolution 
as part of the essential structure of zoology, may be said to 
have completely revolutionized that science; and there is 
scarcely a text-book treating of the subject, howeyer element- 
ary, which has not been molded in accordance with these 
guiding lines of thought. So far as I am aware, this can 
not be said of a single book on the subject of physiology. 
Feeling, therefore, that the time had come for the appearance 
of a work which should attempt to do, in some degree at 
least, for physiology what has been so well done for morphol- 
ogy, the present task was undertaken. But there were other 
changes which it seemed desirable to make. I think any one 
who will examine the methods and reasoning of the physi- 
ology of the day will not fail, on close scrutiny, to recognize 
a tendency to speak of certain conclusions, for various organs 
(and functions), as though they applied to these organs in 
whatever group of animals found, or, at all events, for man, 
no matter what the species of the animal that had been ex- 
perimented upon. For some years I have, in publications of 
my own original researches, strongly protested against such 
methods as illogical. I am wholly at a loss to understand 
how a work, built upon the most fragmentary and hetero- 
geneous evidence, derived from experiments on a few groups 
of animals, or a certain amount of human clinical or patho- 
logical evidence, can be fittingly termed a treatise on " human 
physiology." It will scarcely be denied that conclusions such 


as this method implies would not be tolerated in the subject 
of morphology. 

While in the present work what is strictly applicable to 
other animals and to man has not always been kept apart, 
an effort has been made throughout to be cautious in all the 
conclusions drawn — a state of mind warranted by the past 
history and the present tendencies of physiology. Until our 
laboratory methods become more perfected, the comparative 
method more extensively applied, and conclusions drawn from 
"experiments" modified by comparison with the results of 
clinical, pathological, and all other available sources of infor- 
mation, I feel convinced that we are called upon to teach 
cautiously and modestly. 

Treating, as we do in our books, each subject in a separate 
chapter, there is, as I know by observation, the greatest danger 
that the student may get the idea that each function of the 
body is discharged very much independently ; accordingly, 
there has been throughout a most persistent effort made to 
impress the necessity for ever remembering the absolute de- 
pendence of all parts. Unless this be thoroughly infused 
into a student, it is impossible that he can ever vmderstand 
the wide world of natural objects, or the narrower one of un- 
natural (in a sense) organisms, as see^ in the hospital ward. 

Recognizing how important it is to teach the young stu- 
dent to become an observer and an investigator in spirit and 
in some degree in fact, only such treatment of elaborate 
methods has been introduced as will enable him to form a 
general acquaintance with the modes in which laboratory 
work is carried on, while simple ways of verifying the essen- 
tial truths of physiology have been constantly brought before 
him. As to how far these are actually carried out will de- 
pend not a little on the teacher. The student who learns thus 
to observe and to verify will not fail to apply the method in 
his future career, whatever that may be — whether medical or 
other— nor is he so likely to throw his physiology overboard 
as a useless cargo as soon as his primary examination has 
been passed. 


By frequently calling attention, as has been done through- 
out, to actually discovered or possible differences in function 
for different groups of animals, it is believed that the student 
will become possessed of a spirit of caution in drawing con- 
clusions that will fit him the better for the hospital ward in 
another respect, viz., that he will be prepared for those indi- 
vidual differences actually existing, and which seem to have 
been largely ignored in so many works on physiology, with 
the natural consequence that the student, not finding his 
physiology squaring with the .facts of the clinique, and not 
being prepared for the situation, the result is disappointment 
and disgust, instead of the actual continuation of the study, 
especially as human physiology. 

With a view of widening the student's field of vision, sec- 
tions, under the heading " Special Considerations," have been 
introduced, which it is hoped will not fail to interest and 

Most teachers of experience will welcome the summai;y 
with which each chapter concludes. In connection with no 
subject perhaps can the art of generalizing be better taught 
than with physiology, and to this end these brief synoptical 
sections will, it is thought, prove helpful. 

Systematic instruction in either macroscopic or microscopic 
anatomy has not been undertaken — ^in fact, can not be at- 
tempted, it is believed, except at the expense of physiology 
proper — in a work of moderate compass. At the same time 
attention has been called to those points which have a special 
bearing on each function, and a number of illustrations have 
been inserted with this object in view. 

The introduction of the subject of development at so early 
a stage is a departure that calls for a word o£ explanation. 
An attempt has been made to use embryological facts to 
throw light upon the different functions of the body, and 
especially their relations and interdependence. It therefore 
became necessary to treat the subject early. It is expected, 
however, that the student will return to it after reading the 
remaining chapters of the work. 


As SO large a proportion of those wlio enter upon the 
study of medicine begin their career without any adequate 
preparation in general biology, the subject, as presented in 
this work, will, let me hope, meet an actual need, and prove 
helpful in attaining a broad and sound view of the special 
doctrines of biology. 

It is scarcely necessary to remark that clinical and path- 
ological facts have not been introduced with the view of 
teaching either clinical medicine or pathology, but to indi- 
cate to the student how his physiology bears on his profes- 
sion, and how the above-mentioned subjects throw light upon 
physiology proper and lend interest to that subject. 

My aim has been to make the book, from iirst to last, 
educative; and, retaining a vivid recollection of the severe 
strain put upon the memory of the medical student by our 
present method of crowding so much into at most four years 
of study, an attempt has been made to avoid overloading the 
book with mere facts or technical details, as well as to pre- 
sent the whole subject in as succinct a form as is compatible 
with clearness. Recognizing, too, the very shifting character 
of physiological theories, the latter have generally been pretty 
well kept apart from the actual facts. 

It is hoped that the abundance of the illustrations will 
prove more acceptable than would lengthy treatment of sub- 
jects in the text, for, if the matter of a book is to be digested 
and assimilated, either by the student of general biology or 
by the hard- worked medical student, it must not be bulky. 

The illustrations have been chosen from the best available 
sources, and the authorship of each one duly acknowledged 
in the body of the work. Several original diagrams, such as 
I find exceedingly useful in my own lectures, have been in- 

This book is really an embodiment of my own course of 
lectures, as given during the past two years more especially, 
and with the highest satisfaction, I think it may be said, to 
both students and teacher. 

I have unbounded confidence in the plan of the work, and 


I trust that its newness may excuse, to some degree, any 
shortcomings in the execution. Such a book has become a 
necessity to myself, and it is hoped will be welcomed by 
others. I trust the work may prove suitable, not only for 
the student of human medicine, but for the increasing num- 
ber of students of comparative or veterinary medicine, who 
may desire a broad basis for the study of disease in the 
various animals they are called upon to treat. I have en- 
deavored to make the work specially acceptable to the stu- 
dent of general biology. 

It only remains for me to crave the indulgence of all 
readers, and to thank my publishers, Messrs. D. Appleton & 
Co., for their uniform courtesy and the great pains they have 
taken to present the work in worthy form. 

Wesley Mills. 

Physiological Laboratory, McGill University, 
Montreal, September, 1889. 



General Biology 1 

Introduction . . 1 

Tabular statement of the subdivisions of Biology .... 4 

The Cell 6 

Animal and vegetable cells 5 

Structure of cells 5 

Cell-contents 7 

The nucleus 7 

Tissues 8 

Summary 8 

Unicellular Organisms (Vegetable) 9 

1. Yeast • . . . 9 

Morphological 9 

Chemical 10 

Physiological 10 

Conclusions 10 

3. Protococcus 11 

Morphological 11 

Physiological 11 

Conclusions 12 

Unicellular Animals 12 

The proteus animalcule 13 

Morphological 13 

Physiological 13 

Conclusions 14 

Parasitic Organisms 15 

Fungi 15 

Mucor mucedo . . 15 

The Bacteria 18 

Unicellular Animals with Differentiation of Structure ... 30 

The bell-animalcule 30 

Structure ' 20 

Functions 31 

Multicellular Organisms 33 

The fresh-water polyps 22 

The Cell reconsidered 26 

The Animal Body — an epitomized account of the functions of a mammal . 27 



Living and Lifeless Matter — General explanation and comparison of 

their properties 31 

Classification of the Animal Kingdom 33 

Tabular statement 35 

Man's place in the animal kingdom 35 

The Law op Periodicity or Rhythm in Nature — Explanations and illus- 
trations 36 

The Law of Habit 40 

Its foundation 40 

Instincts 41 

The Origin of the Forms of Life 41 

Arguments from : 

Morphology 43 

Embryology 43 

Mimicry 43 

Kudimentary organs . 43 

Geographical distribution 45 

Paleontology 45 

Fossil and existing species . . . , 45 

Progression 46 

Domesticated animals ^ . .46 

Summary 47 

Reproduction 50 

General I 50 

The ovum 54 

The origin and development of the ovum 57 

Changes in the ovum itself 59 

The male cell 60 

The origin of the spermatozoon 61 

Fertilization of the ovum 62 

Segmentation and subsequent changes ....... 63 

The gastrula 66 

The hen's egg 67 

The origin of the fowl's egg ......... 68 

Embryonic membranes of birds 72 

The foetal (embryonic) membranes of mammals 76 

The placenta 80 

The discoidal placenta 81 

The metadiscoidal placenta 81 

The zonary placenta ^ . . .86 

The difEuse placenta 86 

The polycotyledonary placenta 86 

Microscopic structure of the placenta 87 

Illustrations 87 

Evolution 89 

Summary gg 

The Development of the Embryo Itself 90 

Germ-layers 93 

Origin of the vascular system 97 

The growth of the embryo 102 



Development of the Vascular System in Vertebrates .... 103 

The later stages of the fcetal circulation 103 

Development of the Ueogenital System 106 

The Physiological Aspects of Development 112 

Menstruation and ovulation 113 

The nutrition of the ovum 115 

The foetal circulation 118 

Parturition 120 

Changes in the circulation at birth 120 

Sexual coitus 121 

Organic Evolution reconsidered 127 

Different theories criticised — new views 127 

The Chemical Constitution of the Animal Body 135 

Proximate principles 137 

General characters of proteids 138 

Certain non-crystalline bodies 138 

The fats 139 

Peculiar fats 140 

Carbohydrates 140 

Nitrogenous metabolites 140 

Non-nitrogenous metabolites 141 

Physiological Eeseaeoh and Physiological Reasoning .... 141 

The Blood 147 

Comparative 148 

Corpuscles 149 

History of the blood-cells 151 

Chemical composition of the blood 154 

Composition of serum 155 

Composition of the corpuscles 155 

The quantity and distribution of the blood 156 

The coagulation of the blood 157 

Clinical and pathological 163 

Summary 165 

The Contractile Tissues 166 

General 166 

Comparative 167 

Ciliary movements .■ 168 

The irritability of muscle and nerve 169 

Applications of the Graphic Method to the Study of Muscle 

Physiology . . 171 

Chronographs and various kinds of apparatus .... 171-174 

A single muscular contraction 178 

Tetanic contraction 182 

The muscle-tone 184 

The strength of the stimulus 185 

The changes in a muscle during contraction 186 

The elasticity of muscle 187 

The electrical phenomena of muscle 188 

Chemical changes in muscle 192 

Thermal changes in the contracting muscle 195 



The physiology of nerve 196 

Eleetrotonus 196 

Pathological and clinical 198 

Law of contraction 198 

Electrical organs 199 

Muscular work 199 

Circumstances influencing the character of muscular and nervous activity 200 

The influence, of blood-supply and fatigue 300 

Separation of muscle from the central nervous system .... 303 

The influence of temperature 303 

The intimate nature of muscular and nervous action .... 303 

Unstriped muscle 304 

General 304 

Comparative 305 

Special considerations 305 

Functional variations 307 

Summary of the physiology of muscle and nerve 308 

The Nervous System — General Considerations 310 

Experimental 213 

Automatism 214 

Conclusions 214 

Nervous inhibition ' 315 

The Circulation op the Blood 316 

General 316 

The mammalian heart 317 

Circulation in the mammal 221 

The action of the mammalian heart 333 

The velocity of the blood and blood-pressure 234 

General . . 234 

Comparative 225 

The circulation under the microscope ; 226 

The characters of the blood-flow 337 

Blood-pressure 238 

The Heart 282 

The cardiac movements 232 

The impulse of the heart . 233 

Investigation of the heart-beat from within 284 

The cardiac sounds 235 

Causes of the sounds . . .• . " 286 

Endo-cardiac pressures 338 

The work of the heart 341 

Variations in the cardiac pulsation 342 

The pulse . . 244 

Features of an arterial pulse-tracing 247 

Venous pulse 251 

Pathological 251 

Comparative 251 

The beat of the heart and its modifications 261 

The nervous system in relation to the heart 261 

Influence of the vagus nerve on the heart 265 



Conclusions 269 

The accelerator nerves of the heart 370 

Human physiology 273 

The heart in relation to blood-pressure 274 

The influence ot the quantity of blood 275 

Conclusions . . 377 

The capillaries 281 

Special considerations 382 

Pathological "... 282 

Personal observations 383 

Comparative 383 

Evolution 385 

Summary of the physiology of the circulation 386 

Digestion of Pood 390 

Foodstuffs, milk, etc 390 

Embryological 395 

Comparative 396 

The digestive juices 306 

Saliva and its action 306 

Secretion of the different glands 307 

Comparative 308 

Pathological 308 

Gastric juice 308 

Bile . 311 

General 311 

Pigments 313 

Digestive action , . . 313 

Comparative . . 314 

Pancreatic secretion 314 

Siiccus entericus 317 

Comparative 319 

Secretion as a physiological process . , ' 319 

Secretion of the salivary glands ........ 319 

Secretion by the stomach 333 

The secretion of bile and pancreatic juice 323 

The nature of the act of secretion 326 

Self-digestion of the digestive organs 329 

Comparative 330 

The movements of the digestive organs 331 

Deglutition 332 

Comparative . 335 

The movements of the stomach 335 

Comparative 336 

Pathological 336 

The intestinal movements 337 

Defecation 337 

Vomiting 338 

Comparative 339 

Pathological 339 

The removal of digestive products from the alimentary canal . . . 341 




Lymph and chyle °^ 

The movements of the lymph— comparatiye 342 

Pathological ^^^ 

PiEces ^^^ 

Pathological 354 

The changes produced in the food in the alimentary canal . ... 355 

General 355 

Comparative "^' 

Pathological 357 

Special considerations 358 

Various 358 

Human physiology 362 

Evolution . . 863 

Summary • • 364 

The Respiratory System 365 

General 365 

Anatomical 368 

The entrance and exit of air ..... . ... 369 

The muscles of respiration 372 

Types of respiration 373 

Personal observation 374 

Comparative 375 

The quantity of air respired 378 

The respiratory rhythm 379 

General 379 

Pathological 379 

Respiratory sounds . 381 

Comparison of the inspired and the expired air 381 

Respiration in the blood 383 

Hsemoglobin and its derivatives 385 

General 385 

Blood-spectra .... 387 

Comparative 389 

The nitrogen and the carbon dioxide of the blood . . . 389 

Foreign gases and respiration 392 

Respiration in the tissues ' . . . . 392 

The nervous system in relation to respiration 393 

Nerves and centers concerned 395 

The influence of the condition of the blood on respiration . . . 397 

The Cheyne-Stokes respiration 398 

The effects of variations on the atmospheric pressure 399 

The influence of respiration on the circulation 400 

General 400 

Comparative . . 402 

The respiration and circulation in asphyxia .... . 404 

Pathological 406 

Peculiar respiratory movements . 406 

Coughing, laughing, etc . 406 

Comparative . 407 

Special considerations 408 



Pathological and chemical 408 

Personal observation 408 

Evolution 409 

Summary of the physiology of respiration 410 

Protective and Excretory Functions of the Skin 412 

General 412 

Comparative 413 

The excretory function of the skin 415 

Normal sweat 415 

Pathological 415 

Comparative — Respiration by the skin 415 

Death from suppression of the functions of the skin .... 416 

The excretion of perspiration 416 

Experimental 416 

Human physiology 417 

Absorption by the skin ^ . . . . 418 

Comparative 418 

Summary 418 

Excretion by the Kidney 419 

Anatomical 419 

Comparative 419 

Urine considered physically and chemically 422 

Specific gravity 422 

Color 423 

Reaction 423 

Quantity 423 

Composition : Nitrogenous crystalline bodies 423 

Non-nitrogenous organic bodies 424 

Inorganic salts 424 

Abnormal urine 425 

Comparative 425 

The secretion of urine 426 

Methods of investigation 436 

Theories of secretion 427 

Nervous influence 428 

Pathological 428 

The expulsion of urine 429 

General 429 

Facts of experiment and of experience 429 

Pathological 430 

Comparative 4S0 

Summary of urine and the functions of the kidneys 430 

The Metabolism of the Body 431 

General remarks 431 

The metabolism of the liver . . 432 

The glycogenic function 432 

The uses of glycogen ... 434 

Pathological 435 

Metabolism of the spleen . . 436 

Histological 436 



Chemical . 437 

Spleen curves 439 

The nervous system in relation to the spleen 439 

The construction of fat 440 

General and experimental 440 

Histological 441 

Changes in the cells of the mammary gland 442 

Milk and colostrum 443 

Nature of fat-formation 444 

Pathological 445 

Comparative 445 

The metabolic processes concerned in the formation of urea, uiic acid, 

hippuric acid, and allied bodies 446 

General discussion 446 

Pathological 448 

Evolution 448 

The study of the metabolic processes by other methods .... 449 

Various tabular statements 450 

Starvation and its lessons . 450 

Comparative . 452 

Diets . ., 452 

Feeding experiments 454 

General 454 

Proteid metabolism . . 455 

Nitrogenous equilibrium ... 456 

Comparative 455 

The effects of gelatine in the diet 457 

Pat and carbohydrates 457 

Comparative 458 

The effects of salts, water, etc., on the diet 458 

Pathological 459 

The energy of the animal body 459 

Tabular statements 460 

The sources of muscular energy 461 

Animal heat 46i 

General ._ _ ^i 

Comparative 46j 

The regulation of temperature 464 

Cold-blooded and warm-blooded animals compared .... 465 

Theories of heat formation and heat regulation 466 

Pathological 467 

Special considerations 467 

Evolution 468 

Hibernation 4i^q 

Daily variations in temperature in man and other mammals . . 470 

The influence of the nervous system on metabolism (nutrition) . . .471 

Experimental facts 47I 

Discussion of their significance 473 

General considerations, chemical and pathological .... 476 

Summary of metabolism 476 



The Spinal Coed — General 480 

General 480 

Anatomical . 482 

The reflex functions of the spinal cord 484 

General and experimental 484 

Evolution and heredity 485 

Inhibition of reflexes . 485 

Reflex time 486 

The spinal cord as a conductor of impulses . . . . . . . 487 

Anatomical 488 

Decussation 489 

Pcthological 490 

Paths of impulses 491 

The automatic functions of the spinal cord 493 

General 493 

Spinal phenomena 493 

Special considerations 495 

Comparative 495 

Evolution 496 

Sjmoptical 497 

The Brain 498 

General and anatomical 498 

Animals deprived of their cerebrum 500 

Behavior of various animals and its significance 500 

Have the semicircular canals a co-ordinary function ? . . . . 502 

Experimental, etc 502 

Discussion of the phenomena 502 

Forced movements 503 

Functions of the cerebral convolutions 504 

Comparative 505 

Individual differences in brains 518 

The connection of one part of the brain with another .... 518 

The cerebral cortex 521 

Theories of different observers 522 

The circulation in the brain 525 

Sleep — hibernation — dreaming 526 

Hypnotism — catalepsy — somnambulism 528 

Pathological 580 

Cerebral localization reconsidered 530 

Illustrations of localization 535 

Different methods criticised 535 

Cerebral time 535 

Functions of other portions of the brain 536 

The corpus striatum and the optic thalamus 536 

Corpora quadrigemina 539 

The cerebellum 541 

Pathological 541 

Crura cerebri and and pons Varolii 541 

Pathological -542 

Medulla oblongata 542 



Special considerations 543 

Bmbryological 542 

Evolution 543 

Synoptical 547 

General Remarks on the Senses 548 

Anatomical 548 

General principles 550 

The Skin as an Organ of Sense 551 

General 551 

Pathological 553 

Pressure sensations 554 

Thermal sensations 554 

Tactile sensibility 555 

The muscular sense 557 

General 557 

Pathological 557 

Comparative 558 

Synoptical 559 

Vision 559 

Physical 559 

Anatomical . ' 561 

Bmbryological 562 

Dioptrics of vision 563 

Accommodation of the eye 563 

Alterations in the size of the pupil 569 

Phenomena and their explanations 570 

Pathological 573 

Optical imperfections of the eye . . 572 

Spherical aberration 572 

Astigmatism 573 

Chromatic aberration , 673 

Entoptic phenomena 574 

Anomalies of refraction 574 

Visual sensations 576 

General 576 

Affections of the retina 578 

The nature of the processes which originate visual impulses . 580 

The laws of retinal stimulation ; . 581 

The visual angle .... 583 

Color sensations 583 

Theories of color- vision 584 

Color-blindness . . 585 

Psychological aspects oi vision 586 

The visual field 586 

Imperfections of visual perceptions as '■ iiTadiation," etc. . . . 587 

Influence of the pigment of the macula lutea 589 

After-images, etc 589 

Misconceptions as to the comparative size of objects .... 590 

Subjective phenomena ggj^ 

Co-ordination of the two eyes in vision 59I 



The visual axes 591 

Ocular movements 592 

The horopter 594 

Estimation of the size and distance of objects 595 

Solidity 595 

Protective mechanisms of the eye 596 

Special considerations 598 

Comparative 597 

Evolution 600 

Pathological 602 

Brief synopsis of the physiology of vision 602 

Heabino 604 

General 604 

Anatomical 605 

The membrana tympani 606 

The auditory ossicles 607 

Muscles of the middle ear 608 

The Eustachian tube 609 

Pathological 609 

Auditory impulses 610 

Auditory sensations, perceptions, judgments 615 

General 615 

Auditory judgment 615 

Range of auditory discrimination . . 616 

Special considerations 616 

Comparative 616 

Evolution 618. 

Synopsis of the physiology of hearing 620 

The Sense of Smell 620 

Anatomical 620 

General 621 

Comparative 623 

The Sense of Taste 623 

Anatomical 623 

General 623 

Experimental 623 

Pathological 625 

Comparative 626 

The Cerebbo-Spinal System op Nerves 626 

1. Spinal nerves 636 

General .... 626 

Exception 627 

Additional experiments 627 

Pathological 627 

2. The cranial nerves 628 

General 628 

The motor-oculi, or third nerve 628 

The trochlear, or fourth nerve 629 

The abduoens, or sixth nerve 639 

The facial, or seventh nerve . 639 



The trigeminus, or fifth nerve 630 

The glosso-pharyngeal, or ninth nerve 633 

The pneumogastrie, or tenth nerve 633 

The spinal accessory, or eleventh nerve 635 

The hypoglossal, or tv? elf th nerve 635 

Relations of the cerebro-spinal and sympathetic systems .... 636 

Recent views on this subject 636 

The Voice and Speech 689 

Physical 639 

Anatomical 640 

Laryngoscopic observations 648 

Voice-formation 643 

The registers and the falsetto- voice . 644 

Pathological 646 

Comparative 647 

Speech 649 

General 649 

Formation of vowels and consonants 650 

Whispering 650 

Classification of consonants 651 

Pathological 651 

Special considerations 653 

Evolution 652 

Summary 653 

Locomotion 655 

Anatomical 655 

Mechanical 655 

Standing 656 

Walking 657 

Running 659 

Jumping 659 

Hopping 659 

Comparative : the gait of quadrupeds 659 

Evolution 663 

Man considered physiologically at the Different Periods of his 

Existence 668 

Size and growth 663 

Digestive system 664 

Circulatory and respiratory systems 664 

Dentition 665 

Nervous system ... 666 

Puberty 666 

The sexes 667 

Old age 667 

Comparative 668 

Death 668 

Appendix : Animal Chemistry 671 

Index 691 



Biology (/3ios, life; A.oyos, a dissertation) is the science 
■vrMch treats of the nature of living things; and, since the 
properties of plants and animals can not be explained "vyithout 
some knowledge of their form, this science includes morphol- 
ogy (/top^i/, form ; Xoyos, a dissertation) as well as physiology 
{tfivtris, nature ; Aoyos). 

Morphology describes the various forms of living things 
and their parts ; physiology, their action or function. 

General biology treats neither of animals nor plants exclu- 
sively. Its province is neither zoology nor botany ; but it at- 
tempts to define what is common to all living things. Its aim 
is to determine the properties of organic beings as such, rather 
than to classify or to give an exhaustive account of either ani- 
mals or plants. Manifestly, before this can be done, living 
things, both animal and vegetable, must be carefully compared, 
otherwise it would be impossible to recognize differences and 
resemblances ; in other words, to ascertain what they have in 

When only the highest animals and plants are contem- 
plated, the differences between them seem so vast that they 
appear to have, at first sight, nothing in common but that they 
are living : between a tree and a dog an infant can discrimi- 
nate; but there are microscopic forms of life that thus far 
defy the most learned to say whether they belong to the ani- 
mal or the vegetable world. As we descend in the organic 
series, the lines of distinction grow fainter, till they seem 
finally to all but disappear. 

But let us first inquire : What are the determining charac- 


teristics of living things as such ? By what barriers are the 
animate and inanimate worlds separated ? To decide this, 
falls within the province of general biology. 

Living things grow by interstitial additions of particles of 
matter derived from, without and transformed into their own 
substance, while inanimate bodies increase in size by superfi- 
cial additions of matter over which they have no power of 
decomposition and recomposition so as to make them like 
themselves. Among lifeless objects, crystals approach near- 
est to living forms ; but the crystal builds itself up only from 
material in solution of the same chemical composition as itself. 

The chenaical constitution of living objects is peculiar. Car- 
bon, hydrogen, oxygen, and nitrogen are combined into a very 
complex whole or molecule, as protein; and, when in com- 
bination with a largQ proportion of water, constitute the basis 
of all life, animal and vegetable, known as protoplasm. Only 
living things can manufacture this substance, or even protein. 

Again, in the very nature of the case, protoplasm is con- 
tinually wasting by a process of oxidation, and being built up 
from simpler chemical forms. Carbon dioxide is an invariable 
product of this waste and oxidation, while the rest of the car- 
bon, the hydrogen, oxygen, and nitrogen are given back to the 
inorganic kingdom in simpler forms of combination than those 
in which they exist in living beings. It will thus be evident 
that, while the flame of life continues to burn, there is constant 
chemical and physical change. Matter is being continuously 
taken from the world of things that are without life, trans- 
formed into living things, and then after a brief existence in 
that form returned to the source from which they were origi- 
nally derived. It is true, all animals require their food in or- 
ganized form — that is, they either feed on animal or plant 
forms; but the latter derive their nourishment from the soil 
and the atmosphere, so that the above statement is a scientific 

Another highly characteristic property of all living things 
is to be sought in their periodic changes and very limited dura- 
tion. Every animal and plant, no matter what its rank in the 
scale of existence, begins in a simple form, passes through a 
series of changes of varying degrees of complexity, and finally 
declines and dies ; which simply means that it rejoins the in- 
animate kingdom: it passes into another world to which it 
formerly belonged. 

Living things alone give rise to living things ; protoplasm 


alone can beget protoplasm; cell begets cell. Omne anvmcH. 
(anima, life) ex ovo applies with a wide interpretation to all 
living forms. 

From what bas been said it will appear that life is a condi- 
tion of ceaseless change. Many of the movements of the pro- 
toplasm composing the cell-units of which living beings are 
made are visible under the microscope ; their united effects are 
open to common observation — as, for example, in the move- 
ments of animals giving rise to locomotion we have the joint 
result of the movements of the protoplasm composing millions 
of muscle-cells. But, beyond the powers of any microscope that 
has been or probably ever will be invented, there are molecular 
movements, ceaseless as the flow of time itself. All the processes 
which make up the life-history of organisms involve this mo- 
lecular motion. The ebb and flow of the tide may symbolize 
the influx and efflux of the things that belong to the inanimate 
world, into and out of the things that live. 

It follows from this essential instability in living forms that 
life must involve a constant struggle against forces that tend 
to destroy it ; at best this contest is maintained successfully for 
but a few years in all the highest grades of being. So long as 
a certain equilibrium can be maintained, so long may life con- 
tinue and no longer. 

The truths stated above will be illustrated in the simpler 
forms of plants and animals in the ensuing pages, and will be- 
come clearer as each chapter of this work is perused. They 
form the fundamental laws of general biology, and may be 
formulated as follows : 

1. Living matter or protoplasm is characterized by its chem- 
ical composition, being made up of carbon, hydrogen, oxygen, 
and nitrogen, arranged into a very complex molecule. 

2. Its universal and constant waste and its repair by inter- 
stitial formation of new matter similar to the old. 

3. Its power to give rise to new forms similar to the parent 
ones by a process of division. 

4. Its manifestation of periodic changes constituting devel- 
opment, decay, and death. 

Though there is little in relation to living beings which 
may not be appropriately set down under zoology or botany, it 
tends to breadth to have a science of general biology which 
deals with the properties of things simply as living, irrespective 
very much as to whether they belong to the realm of animals 
or plants. The relation of the sciences which may be regarded 


•as subdivisions of general biology is well shown in the follow- 
ing table : * 

The science of structure ; the 
term being usually ap- 
plied to the coarser and 
more obvious composition 
of plants or animals. 




of liv- 

things ; 
i. e., of 
in the 










Microscopical anatomy. 
The ultimate optical an- 
alysis of structure by the 
aid of the microscope; 
separated from anatomy 
only as a matter of con- 

The classification of living 
things, based chiefly on 
phenomena of structure. 

Considers the position of liv- 
ing things in space and 
time ; their distribution 
over the present face of 
the earth ; and their distri- 
bution and succession at 
former periods, ps dis- 
played in fossil remains. 

The science of development 
from the germ; includes 
many mixed problems 
pertaining both to mor- 
phology and physiology. 
At present largely mor- 

The special science of the 
functions of the individ- 
ual in health and in dis- 
ease ; hence including 

The science of mental phe- 

The science of social life, 
i. e., the life of communi- 
ties, whether of men or 
of lower animals. 


of veg- 




or ani- 


of liv- 
things ; 
i. e., of 
in the 

* Taken from the " General Biology " of Sedgwick and Wilson. 



All living tilings, great and small, are composed of cells. 
Animals may be divided into those consisting of a single cell 
(Protozoa), and tliose made up of a multitude of cells {Metazoa) ; 
but in every case the animal begins as a single cell or ovum 
from which all the other cells, however different finally from 
one another either in form or function, are derived by processes 
of growth and division ; and, as will be seen later, the whole 
organism is at one period made up of cells practically alike in 
structure and behavior. The history of each individual animal 
or plant is the resultant of the conjoint histories of each of its 
cells, as that of a nation is, when complete, the story of the 
total outcome of the lives of the individuals composing it. 

It becomes, therefore, highly important that a clear notion 
of the characters of the cell be obtained at the outset; and 
this chapter will be devoted to presenting a general account of 
the cell. 

The cell, whether animal or vegetable, in its most complete 
form consists of a mass of viscid, semifluid, transparent sub- 
stance {protoplasm), a cell wall, and a more or less circular body 
{nudeus) situated generally centrally within ; in which, again, 
is found a similar structure {nucleolus). 

This description applies to both the vegetable and the ani- 
mal cell ; but the student will find that the greater proportion 
of animal cells have no cell wall, and that very few vegetable 
cells are without it. But there is this great difference between 
the animal and vegetable cell : the former never has a cellulose 
wall, while the latter rarely lacks such a covering. In every 
case the cell wall, whether in animal or vegetable cells, is of 
greater consistence than the rest of the cell. This is especially 
true of the vegetable cell. 

It is doubtful whether there are any cells without a nucleus, 
while not a few, especially when young and most active, pos- 
sess several. The circular form may be regarded as the typical 
form of both cells and nuclei, and their infinite variety in size 
and form may be considered as in great part the result of the 
action of mechanical forces, such as mutual pressure; this is, 
of course, more especially true of shape. Reduced to its great- 
est simplicity, then, the cell may be simply a mass of protoplasm 
with a nucleus. 

* The illustrations of the sections following will enable the student to form a 
generalized mental picture of the cell in all its parts. 


It seems probable tbat the mimerous researcbes of recent 
years and otbers now in progress will open up a new world of 
cell biology wbicb will greatly advance our knowledge, espe- 
cially in tbe direction of increased depth and accuracy. 

Tbougb many points are still in dispute, it may be safely 
said tbat the nucleus plays, in most cells, a role of the highest 
importance ; in fact, it seems as though we might regard the 
nucleus as the directive brain, so to speak, of the individual 
cell. It frequently happens that the behavior of the body of 
the cell is foreshadowed by that of the nucleus. Thus fre- 

FiG. 1.— NncLEATi DIVISION. A-H, karjrokinesis ot a tissue-cell. A, nuclear reticulum in its or- 
dinary state. B, preparing for division ; the contour is less defined, and the fibers thicker 
and less Intricate. C, wreath -stage; the chromatin is arranged in a complicated looping 
round the equator of the achromatin spindle. D, monaster-stage ; the chromatin now 
appears as centripetal equatorial Vs, each ot which should be represented as double. 

E, a migration of the half of each chromatin loop towards opposite poles of the spindle. 

F, diaster-stage ; the chromatin forms a star, round each pole of a spindle, each aster be- 
ing connected by strands of achromatin. G, daughter-wreath stage ; the'newly formed 
nuclei are passing through their retrogressive development, which is completed in the 
resting stage, H. d-/, karyokinesis of an egg-cell, showing the smaller amount of chro- 
matin than in the tissue-cell. The stages d, e, /, correspond to D, E, F, respectively. The 
polar star at the end of the spindle Is composed of protoplasm-granules or the cell itself, 
and must not be mistaken for the diaster (F|. The coarse lines represent the chromatin, 
the fine lines the achromatin, and the dotted lines cell-granules. (Chiefly modified from 
Flemming.) X-Z, direct nuclear division in the cells of the embryonic integument of the 
Em'opean scorpion. After Blochmann {Haddon). 

quently, if not always, division of the body of the nucleus pre- 
cedes that of the cell itself, and is of a most complicated char- 
acter {karyoMnesis or mitosis). The cell wall is of subordinate 
importance in the processes of life, though of great value as a 
mechanical support to the protoplasm of the cell and the aggre- 


gations of cells known as tissues. Tlie greater part of a tree 
may be said to be made up of the thickened walls of the cells, 
and these are destitute of true vitality, unless of the lowest 
order ; while the really active, growing part of an old and large 
tree constitutes but a small and limited zone, as may be learned 
from the plates of a work on modern botany representing sec- 
tions of the wood. 

Animals, too, have their rigid parts, in the adult state espe- 
cially, resulting from the thickening of a part or the whole of 
the cell by a deposition usually of salts of lime, as in the case of 
the bones of animals. But ia some cases, as in cartilage, the 
cell wall or capsule undergoes thickening and consolidation, 
and several may fuse together, constituting a matrix, which is 
also made up in part, possibly, of a secretion from the cell pro- 
toplasm. In the outer parts of the body of animals we have a 
great abundance of examples of thickening and hardening of 
cells. Very well known instances are the indurated patches of 
skin {epithelium) on the palms of the hands and elsewhere. 

It will be scarcely necessary to remark that in cells thus 
altered the mechanical has largely taken the place of the vital 
in function. This at once harmonizes with and explains what 
is a matter of common observation, that old men are less active 
— have less of life within them, in a word, than the young. 
Chemically, the cellulose wall of plant-cells consists of carbon, 
hydrogen, and oxygen, in the same relative proportion as exists 
in starch, though its properties are very different from those of 
that substance. 

Turning to cell contents, we find them everywhere made up 
of a clear, viscid substance, containing almost always granules 
of varying but very minute size, and differing in consistence, 
not only in different groups of cells, but often in the same cell, 
so that we can distinguish an outer portion (ectoplasm) and an 
inner more fluid and more granular region (endoplasm,). 

The nucleus is a body with very clearly defined outline (in 
some cases limited by a membrane), through which an irregu- 
lar network of • fibers extends that stains more deeply than any 
other part of the whole cell. 

Owing to the fact that it is so readily changed by the action 
of reagents, it is impossible to ascertain the exact chemical com- 
position of living protoplasm; in consequence, we can only 
infer its chemical structure, etc., from the examination of the 
dead substance. 

In general, it may be said that protoplasm belongs to the 


class of bodies known as proteids — that is, it consists chemically 
of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar- 
ranged into a very complex and unstable molecule. This very 
instability seems to explain at once its adaptability for the 
manifestation of its nature as living matter, and at the same 
time the readiness with which it is modified by many circum- 
st9,nceS , so that it is possible to understand that life demands 
an incessant adaptation of internal to external conditions. 

It seems highly probable that protoplasm is not a single pro- 
teid substance, but a mixture of such ; or let us rather say, fur- 
nishes these when chemically examined and therefore dead. 

Very frequently, indeed generally, protoplasm contains other 
substances, as salts, fat, starch, chlorophyl, etc. 

From the fact that the nucleus stains differently from the 
cell contents, we may infer a difference between them, physical 
and especially chemical. It (nucleus) furnishes on analysis mi- 
dein, which contains the same elements as protoplasm (with the 
exception of sulphur) together with phosphorus. Nuclei have 
great resisting power to ordinary solvents and even the digest- 
ive juices. 

Inasmuch as all vital phenomena are associated with proto- 
plasm, it has been termed the " physical basis of life " (Hux- 

Tissues. — A collection of cells performing a similar physio- 
logical action constitutes a tissue. 

Generally the cells are held together either by others with 
that sole function, or by cement material secreted by. them- 
selves. An organ may consist of one or several tissues. Thus 
the stomach consists of muscular, serous, connective, and gland- 
ular tissues besides those constituting its blood-vessels, lym- 
phatics, and nerves. But all of the cells of each tissue have, 
speaking generally, the same function. The student is referred 
to works on general anatomy and histology for classifications 
and descriptions of the tissues. 

The statements of this chapter will find illustration in the 
pages immediately following, after which we shall return to 
the subject of the cell afresh. 

Summary.— The typical cell consists of a wall, protoplasmic 
contents, and a nucleus. The vegetable cell has a limiting 
membrane of cellulose. Cells undergo differentiation and may 
be united into groups forming tissues which serve one or more 
definite purposes. 

The chemical constitution of protoplasm is highly complex 



and unstable. The nucleus plays a prominent part in the life- 
Mstory of the cell, and seems to be essential to its perfect devel- 
opment and greatest physiological efficiency. 

Yeast (Tonda, Saccharomyces Cerevisice). 

The essential part of the common substance, yeast, may be 
studied to advantage, as it affords a simple type of a vast group 
of organisms of profound 
interest to the student of 
physiology and medicine. 
To state, first, the main 
facts as ascertained by 
observation and experi- 
ment : 

Uorphological. — The 
particles of which yeast 
is composed are cells of a 
circular or oval form, of 
an average diameter of 
about -j^ of an inch. 

Each individual torula 
cell consists of a trans- 
parent homogeneous cov- 
ering (celliilose) and gran- 
ular semifluid contents 
(protoplasm). Within the 
latter there may be a 
space (vacuole) filled with 
more fluid contents. 

The various cells pro- 
duced bv buddinar mav ^°- S-— Tte endogomdia (ascospore) phase of repro- 
•' , ° , •' ductioa— i. e., endogenous division. 

remain united like strings 

of beads. Collections of 

masses composed of four 

or more subdivisions (as- 

cospores), which finally 

separate by rupture of 

the original cell wall, having thus become themselves inde^ 

pendent cells, may be seen more rarely (endogenous division). 

Fig. 2. — ^Various stages in the development of brewer's 
yeast, seen, with the exception of the first in the 
series, with an ordlnaiy high power (Zeiss, D. 4) of 
the microscope. The first is greatly magnified 
(Gundlach''s t^ immersion lens). The second series 
of four represents stages in the division of a single 
cell ; and the third series a branching colony. 
Everywhere the hght areas indicate vacuoles. 

Fig. 4.— Further development of the forms represented 
in Fig. 3. 


The yeast-cell is now believed to possess a nucleus. 

Chemical. — When yeast is burned and the ashes analyzed, 
they are found to consist chiefly of salts of potassium, calcium, 
and magnesium. 

The elements of which yeast is composed are C, H, O, N, S, 
P, K, Mg, and Ca ; but chiefly the first four. 

FhysiologicaL — If a little of the powder obtained by drying 
yeast at a temperature below blood-heat be added to a solution 
of sugar, and the latter be kept warm, bubbles of carbon di- 
oxide will be evolved, causing the mixture to become frothy ; 
and the fluid will acquire an alcoholic character {fermenta- 

If the mixture be raised to the boiling-point, the process de- 
scribed at once ceases. 

It may be further noticed that in the fermenting saccharine 
solution there is a gradual increase of turbidity. All of these 
changes go on perfectly well in the total absence of sunlight. 

Yeast-cells are found to grow and reproduce abundantly in 
an artificial food solution consisting of a dilute solution of cer- 
tain salts, together with sugar. 

Conclusions. — What are the conclusions which may be legiti- 
mately drawn from the above facts ? 

That the essential part of yeast consists of cells of about the 
size of mammalian blood-corpuscles, but with a limiting wall 
of a substance different from the inclosed contents, which latter 
is composed chiefly of that substance common to all living 
things — protoplasm ; that like other cells they reproduce their 
kind, and in this instance by two methods : gemmation giving 
rise to the bead-like aggregations alluded to above ; and in- 
ternal division of the protoplasm {endogenous division). 

From the circumstances under which growth and reproduc- 
tion take place, it will be seen that the original protoplasm of 
the cells may increase its bulk or grow when supplied with 
suitable food, which is not, as will be learned later, the same in 
all respects as that on which green plants thrive ; and that this 
may occur in darkness. But it is to be especially noted that the 
protoplasm resulting from the action of the living cells is 
wholly different from any of the substances used as food. This 
power to construct protoplasm from inanimate and unorgan- 
ized materials, reproduction, and fermentation are all proper- 
ties characteristic of living organisms alone. 

It will be further observed that these changes all take place 
within narrow limits of temperature; or, to put the matter 


more generally, tliat the life-history of this humble organism 
can only he unfolded under certain well-defined conditions. 

Pkotococcus {Protococcus plv/vialis). 

The study of this one-celled plant will afford instructive 
comparison between the ordinary green plant and the colorless 
plants or fungi. 

Like ToruLa it is selected because of its simple nature, its 
abundance, and the ease with which it may be obtained, for it 
abounds in water-barrels, standing pools, drinking-troughs, etc. 

Morphological — Protococcus consists of a structureless wall 
and viscid granular contents, i. e., of cellulose and protoplasm. 

The protoplasm may contain starch and a red or green color- 
ing matter (cMoropliyl). It probably contains a nucleus. The 
cell is mostly globular in form. 

FiQ. 5. Fio. 6. 

"<• err 

Fig. 7. 

Figs. 5 to 7 represent successive stages observed in the life-history of Protococci scraped from 
the bark of a tree. 

Fig. 5. —A group in the dried state, illustrating method of division. 

Fio. 6.— One of the above after two days' immersion in water. 

Fig. 7. — ^Various phases in the later motile stage assumed by the above specimens. , The nu- 
cleus is denoted by nc ; the cell wall by c,w ; and the coloring-matter by the dark spot. 
On the left of Fig. 7 an individual may De seen that is devoid of a cell walL 

Physiological. — It reproduces by division of the original cell 
(fission) into similar individuals, and by a process of budding 
and constriction (gemmation) which is much rarer. Under the 
influence of sunlight it decomposes carbon dioxide (CO,), fix- 
ing the carbon and setting the oxygen free. It can flourish per- 
fectly in rain-water, which contains only carbon dioxide, salts 
of ammonium, and minute quantities of other soluble salts that 
may as dust have been blown into it. 

There is a motile form of this unicellular plant, and in this 
stage it moves through the fluid in which it lives by means of 


extensions of its protoplasm (cilia) through the cell wall; or 
the cell wall may disappear entirely. Finally, the motile form, 
withdrawing its cilia and clothing itself with a cellulose coat, 
becomes globular and passes into a quiescent state again. 
Much of this part of its history is common to lowly animal 

Conclnsions. — It will be seen that there is much in common 
in the life-history of Torula and Protococcus. By virtue of be- 
ing living protoplasm they transform unorganized material into 
their own substance ; and they grow and reproduce by analo- 
gous methods. 

But there are sharply defined differences. For the green 
plant sunlight is essential, in the presence of which its chloro- 
phyl prepares the atmosphere for animals by the removal of 
carbonic anhydride and the addition of oxygen, while for 
Torula neither this gas nor sunlight is essential. 

Moreover, the fungus (Torula) demands a higher kind of 
food, one more nearly related to the pabulum of animals ; and 
is absolutely independent of sunlight, if not actually idjured 
by it ; not to mention the remarkable process of fermentation. 

The Proteus Animalcule (AmoBba). 

In order to illustrate animal life in its simpler form we 
choose the above-named creature, which is nearly as readily 
obtainable as Protococcus and often under the same circum- 

Morphological. — Amceba is a microscopic mass of transparent 
protoplasm, about the size of the largest of the colorless blood- 
corpuscles of cold-blooded animals, with a clearer, more con- 
sistent outer zone (edosarc), (although without any proper cell 
wall), and a more fluid, granular inner part. A clear space 
(contractile vesicle, vacuole) makes its appearance at intervals in 
the ectosarc, which may disappear somewhat suddenly. This 
appearance and vanishing have suggested the term pulsating 
or contracting vesicle. Both a nucleus and nucleolus may be 
seen in Amoeba. At varying short periods certain parts of its 
body ( pseudopodia) are thrust out and others withdrawn. 

Physiological — Amoeba can not live on such food as proves 
adequate for either Protococcus or Torula, but requires, besides 



inorganic and xmorganized food, also organized matter in the 
form of a complex organic compound known as protein, which 

Fio. 9. 

Fio. 10. 

Fia. 11. 

Fig. 12. 

Fio. 13. 


Fib. 14. 

FiQ. 15. 


Fig. 16. 

Figs. 8 to 16, represent successive phases in the life-history of an Amoeboid organism, Icept 
under constant observation for three days ; Fig. 16 a similar organism encysted, which 
was a few hours later set free by the disintegration of the cyst. (All the figures are 
drawn under Zeiss, D. 3.) 

Fig. 8. — The locomotor phase ; the ectoplasm is seen protruding to form a pseudopodium, into 
which the endoplasm passes. 

Fig. 9. — A stage in the ingestive phase. A vegetable organism, /p, is undergoing intussus- 

Fig. 10.— a portion of the creature represented in Fig. 9 after complete ingestion of the food- 

Figs. 11, 12.— Successive stages in the assimilative and excretory processes. Fig. 18 repre- 
sents the organism some twenty hours later than as seen in Fi^. 11. The undigested rem- 
nants of the ingested organism are represented undergoing ejection (excretion) at fp^ in 
Fig. 12. 

Figs. 13, 14, 15, represent successive stages in the reproductive process of the same individ- 
ual, observed two days later. It willbe noticed (Fig. 13) that the nucleus divides first. 

In the above figures, iic, denotes the contracting vacuole ; no, the nucleus ; ps, pseudopo- 
dium ; dt, diatom ; fp, food-particle. 

contains nitrogen in addition to carbon, hydrogen, and oxygen. 
In fact, Amceba can prey upon both plants and animals, and 
thus use up as food protoplasm itself. The pseudopodia serve 
the double purpose of organs of locomotion and prehension. 
This creature absorbs oxygen and evolves carbon dioxide. 


Inasmuch as any part of the body may serve for the admission, 
and possibly the digestion, of food and the ejection of the use- 
less remains, we are not able to define the functions of special 
parts. Amoeba exercises, however, some degree of choice as to 
what it accepts or rejects. 

The movements of the pseudopodia cease when the tempera- 
ture of the surrounding medium is raised or lowered beyond a 
certain point. It can, however, survive in a quiescent form 
greater depression than elevation of the temperature. Thus, at 
35° C, heat-rigor is induced ; at 40° to 45° C, death results ; but 
though all movement is arrested at the freezing-point of water, 
recovery ensues if the temperature be gradually raised. Its 
form is modified by electric shocks and chemical agents, as well 
as by variations in the temperature. At the present time it is 
not possible to define accurately the functions of the vacuoles 
found in any of the organisms thus far considered. It is 
worthy of note that Amoeba may spontaneously assume a 
spherical form, secrete a structureless covering, and remain in 
this condition for a variable period, reminding us of the similar 
behavior of Torula. 

Amoeba reproduces by fission, in which the nucleiis takes a 
prominent if not a directive part, as seems likely it does in re- 
gard to all the functions of unicellular organisms. 

Conclasions. — It is evident that Amoeba is, in much of its be- 
havior, closely related to both colored and colorless one-celled 
plants. All of the three classes of organisms are composed of 
protoplasm ; each can construct protoplasm out of that which is 
very different from it ; each builds up the inanimate inorganic 
world into itself by virtue of that force which we call vital, but 
which in its essence we do not understand ; each multiplies by 
division of itself, and all can only live, move, and have their 
being under certain definite limitations. But even among 
forms of life so lowly as those we have been considering, the 
differences between the animal and vegetable worlds appear. 
Thus, Amoeba never has a cellulose wall, and can not subsist 
on inorganic food alone. The cellulose wall is not, however, 
invariably present in plants, though this is generally the case ; 
and there are animals (Ascidians) with a cellulose investment. 
Such are very exceptional cases. But the law that animals 
must have organized material (protein) as food is without ex- 
ception, and forms a broad line of distinction between the ani- 
mal and vegetable kingdoms. 

Amoeba will receive further consideration later; in the 


mean time, we turn to the study of forms of life in many respects 
intermediate between plants and animals, and full of practical 
interest for mankind, on account of tlieir relations to disease, 
as revealed by recent investigations. 


The Fungi. 
Molds {PenioUUum Glaucum and Mucor Mucedo). 

Closely related to Torula physiologically, but of more com- 
plex structure, are the molds, of which we select for convenient 
study the common green mold {Penicillium), found growing in 
dark and moist places on bread and similar substances, and the 
white mold (Mucor), which grows readily on manure. 

The fungi originate in spores, which are essentially like 
Torula in structure, by a process of budding and longitudinal 
extension, resulting in the formation of transparent branches 
or tubules, filled with protoplasm and invested by cellulose 
walls, across which transverse partitions are found at regular 
intervals, and in which vacuoles are also visible. 

The spores, when growing thus in a liquid, give rise to up- 
ward branches {aerial hyphce), and downward branches or root- 
lets {submerged hyphce,). These multitudinous branches inter- 
lace in every direction, forming an intricate felt-work, which 
supports the green powder (spores) which may be so easily 
shaken off from a growing mold. In certain cases the aerial 
hyphse terminate in tufts of branches, which, by transverse 
division, become split up into spores {Conidia), each of which 
is similar in structure to a yeast-cell. 

The green coloring matter of the fungi is not chlorophyl. 
The Conidia germinate under the same conditions as Torula. 

Mucor Mucedo. — The growth and development of this mold 
may be studied by simply inverting a glass tumbler over some 
horse-dung on a saucer, into which a very little water has 
been poured, and keeping the preparation in a warm place. 

Very soon whitish filaments, gradually getting stronger, ap- 
pear, and are finally topped by rounded heads or spore-cases 
{Sporangia). These filaments are the hyphce,, similar in struct- 
ure to those of Penicillium. The spore-case is filled with a 
multitude of oval bodies {spores), resulting from the subdivis- 
ion of the protoplasm, which are finally released by the spore- 




FiQ. 19. 

JfiQ. 17. 


Fig. 21. 

Fig. ST. 

Fig. 88. 


Figs. 17 to 28.— In the foUoTring flgTires, ha, denotes aerial hyphse ; «p, ^uranginm ; zy zy- 
gospore ; ex, exosporium ; my, mycelium ; mc, mucilage ; cl, columella ; en, endogomdia. 

Fig. 17. — Spore-bearing hyphsB of Mucor, growing from horse-dung. 

Fig. 18.— The same, teased out with needles (A, 4). 

Figs. 19, 20, 21.— Successive stages in the development of the sporangium. 

Fig. 23.— Isolated spores of Mucor. 

Fig. 23.— Germioatmg spores of the same mold. 

Fig. 24.— Successive stages in the germination of a single spore. 

Figs. 25, 26, 27. — Successive phases in the conjugative process of Mucor. 

Fio. 28.— Successive stages observed during ten hours in the growth of a conidiophore of Peni- 
cilUum in an object-glass culture (D, 4). 

case becoming tkinned to the point of rupture. The devel- 
opment of these spores takes place in substantially the same 
manner as those of PenicilUum. Sporangia developing spores 
in this fashion by division of the protoplasm are termed asci, 
and the spores ascospores. 

So long as nourishment is abundant and the medium of 
growth fluid, this asexual method of reproduction is the only 
one ; but, under other circumstances, a mode of increase, known 
as conjugation, arises. Two adjacent hyphse enlarge at the ex- 
tremities into somewhat globular heads, bend over toward each 
other, and, meeting, their opposed faces become thinned, and 
the contents intermingle. The result of this union {zygospore) 
undergoes now certain further changes, the cellulose coat being 
separated into two — an outer, darker in color {exosporium), and 
an inner colorless one {endosporium). 

Under favoring circumstances these coats burst, and a 
branch sprouts forth from which a vertical tube arises that 
terminates in a sporangium, in which spores arise, as before de- 
scribed. It will be apparent that we have in Mucor the exem- 
plification of what is known in biology as " alternation of gen- 
erations" — ^that is, there is an intermediate generation be- 
tween the original form and that in which the original is 
again reached. 

Physiologically the molds closely resemble yeast, some of 
them, as Mucor, being capable of exciting a fermentation. 

The fungi are of special interest to the medical student, be- 
cause many forms of cutaneous disease are directly associated 
with their growth in the epithelium of the skin, as, for exam- 
ple, common ringworm ; and their great vitality, and the facil- 
ity with which their spores are widely dispersed, explain the 
highly contagious nature of such diseases. The media on which 
they flourish (feed) indicates their great physiological differ- 
ences in this particular from the green plants proper. They are 
closely related in not a few respects to an important class of 
vegetable organisms, known as bacteria, to be considered forth- 




The Bactekia. 

The bacteria include numberless varieties of organisms of 
extreme minuteness, many of them visible only by the help of 
the most powerful lenses. Their size has been estimated at 
from sa^oo to nrW of an inch in diameter. 

They grow mostly in the longitudinal direction, and repro- 
duce by transverse division, forming spores from which new 
generations arise. 

Some of them have vibratile cilia, while the cause of the 
movements of others is quite unknown. 

As in many other lowly forms of life, there is a quiescent 
as well as an active stage. In this stage {zodglo&a form) they 

« «, e 

Fig. 29. 

Fio. 30. 

Fig. 31. 

Fig. 33. 

Fig. 29.— Micrococcus, very like a spore, but usually much smaller. 

Fig. 30.— Bacterium. 

Fig. 31.— Bacillus. The central filament presented this segmented appearance as the result of 

a process of transverse division occurring during ten minutes' observation. 
Fig. 32. — Spirillum ; various forms. The first two represent vibrio, which is possibly only a 

stage of spirillum. 
Fig. 33. — A drop of the surface scum, showing a spirillum aggregate in the resting state. 

are surrounded by a gelatinous matter, probably secreted by 


Bacteria grow and reproduce in Pasteur's solution, render- 
ing it opaque, as well as in almost all fluids that abound in 
proteid matter. That such fluids readily putrefy is owing to 
the presence of bacteria, the vital action of which suffices to 
break asunder complex chemical compounds and produce new 
ones. Some of the bacteria require oxygen, as Bacillus an- 
thracis, while others do not, as the organism of putrefaction. 
Bacterium termo. 

Bacteria are not' so sensitive to slight variations in tempera- 
ture as most other organisms. They can, many of them, with- 
stand freezing and high temperatures. All bacteria and all 
germs of bacteria are killed by boiling water, though the spores 
are much more resistant than the mature organisms themselves. 
Some spores can resist a dry heat of 140° C. 

The spores, like Torula and Protococcus, bear drying, with- 
out loss of vitality, for considerable periods. 

That different groups of bacteria have a somewhat different 
life-history is evident from the fact that the presence of one 
checks the other in the same fluid, and that successive swarms 
of different kinds may flourish where others have ceased to 

That these organisms are enemies of the constituent cells of 
the tissues of the highest mammals has now been abundantly 
demonstrated. That they interfere with the normal working 
of the organism in a great variety of ways is also clear ; and 
certain it is that the harm they do leads to aberration in cell- 
life, however that may be manifested. They rob the tissues of 
their nutriment and oxygen, and poison them by the products 
of the decompositions they produce. But apart from this, their 
very presence as foreign agents must hamper and derange the 
delicate mechanism of cell-life. 

These organisms seem to people the air, land, and waters 
with invisible hosts far more numerous than the forms of life 
we behold. Fortunately, they are not all dangerous to the 
higher forms of mammalian life ; but that a large proportion 
of the diseases which afflict both man and the domestic animals 
are directly caused, in the sense of being invariably associated 
with, the presence of such forms of life, is now beyond doubt. 

The facts stated above explain why that should be so ; why 
certain maladies should be infectious ; how the germs of dis- 
ease may be transported to a friend wrapped up in the folds of 
a letter. 

Disease thus caused, it must not be forgotten, is an illustra- 


tion of the struggle for existence and the survival of the fittest. 
If the cells of an organism are mightier than the bacteria, the 
latter are overwhelmed; but if the bacteria are too great in 
numbers or more vigorous, the cells must yield ; the battle may 
waver — now dangerous disease, now improvement — but in the 
end the strongest in this, as in other instances, prevail. 


The Bell- Animalcule {VorticeUa). 

Amoeba is an example of a one-celled animal with little per- 
ceptible differentiation of structure or corresponding division 
of physiological labor. This is not, however, the case with all 
unicellular animals, and we proceed to study one of these with 
considerable development of both. The Bell - animalcule is 
found in both fresh and salt water, either single or in groups. 
It is anchored to some object by a rope-like stalk of clear pro- 
toplasm, that has a spiral appearance when contracted; and 
which, with a certain degree of regularity, shortens and length- 
ens alternately, suggesting that more definite movement (con- 
traction) of the form of protoplasm known as muscle, to be 
studied later. 

The body of the creature is bell-shaped, hence its name ; the 
bell being provided with a thick everted lip (peristome), covered 
with bristle-like extensions of the protoplasm (cilia), which are 
in almost constant rhythmical motion. Covering the mouth of 
the bell is a lid, attached by a hinge of protoplasm to the body, 
which may be raised or lowered. A wide, funnel-like depres- 
sion (oesophagus) leads into the softer substance within which 
it ends blindly. The outer part of the animal (cuticula) is 
denser and more transparent than any other part of the whole 
creature ; next to this is a portion more granular and of inter- 
mediate transparency between the external and innermost 
portions (cortical layer). Below the disk is a space (contractile 
vesicle) filled with a thin, clear fluid, which may be seen to 
enlarge slowly and then to collapse suddenly. When the Vorti- 
ceUa is feeding, these vesicles may contain food-particles, and 
in the former, apparently, digestion goes on. Such food vacu- 
oles (vesicles) may circulate up one side of the body of the ani- 
mal and down the other. Their exact significance is not known, 
but it would appear as if digestion went on within them ; and 



possibly the clear fluid with which they are filled may be a spe- 
cial secretion with solvent action on food. 


Pio. 37. 

Fis. 38. 

Fios. 34 to 40.— In the following figures d, denotes disc ; 
p, peristome ; re, contractile vacuole ; u/, food- 
vacuole ; vs, vestibule ; c/, contractile fiber ; c, 
cyst ; nc, nucleus ; cl, cilium. 

Fig. 34.— a group of vorticellse showing the creature in 
various positions (A. 3). 

Fia. 3'5.— The same, in the extended and in the retracted 
state. (Surface views.) 

Fig. 36.— Shows food-vacuoles ; oue in the act of inges- 

Fig. 37. — A vorticella, in which the process of multiplica- 
tion by fission is begun. 

Fig. 38.— The results of fission ; the production of two in- 
dividuals of unequal size. 

Fig. 39.— Illustration of reproduction by conjugation. 

Fig. 40.— An encysted vorticella. 

Fig. 35. 

Situated somewhat centrally is a horseshoe-shaped body, 
with well-defined edges, which stains more readily than the rest 
of the cell, indicating a different chemical composition ; and, 
from the prominent part it takes in the reproductive and other 
functions of the creature, it may be considered the nucleus 

Multiplication of the species is either by gemmation or by 
fission. In the first case the nucleus divides and the fragments 
are transformed into locomotive germs ; in the latter the entire 
animal, including the nucleus, divides longitudinally, each half 
becoming a similar complete, independent organism. Still an- 


other method of reproduction is known. A more or less globu- 
lar body encircled with a ring of cilia and of relatively small 
size may sometimes be seen attached to the usual form of Vorti- 
cella, with which it finally becomes blended into one mass. This 
seems to foreshadow the '' sexual conjugation " of higher forms, 
and is of great biological significance. 

Vorticella may pass into an encysted and quiescent stage for 
an indefinite period and again become active. The history of 
the Bell-animalcule is substantially that of a vast variety of 
one-celled organisms known as Infusoria, to which Amoeba 
itself belongs. It will be observed that the resemblance of this 
organism to Amoeba is very great ; it is, however, introduced 
here to illustrate an advance in differentiation of structure ; and 
to show how, with the latter, there is usually a physiological 
advance also, since there is additional functional progress or 
division of labor; but still the whole of the work is done with- 
in one cell. Amoeba and Vorticella are both factories in which 
all of the work is done in one room, but in the latter case the 
machinery is more complex than in the former ; there are cor- 
respondingly more processes, and each is performed with greater 
perfection. Thus, food in the case of the Bell-animalcule is 
swept into the gullet by the currents set up by the multitudes 
of vibrating arms around this opening and its immediate neigh- 
borhood ; the contractile vesicles play a more prominent part ; 
and the waste of undigested food is ejected at a more definite 
portion of the body, the floor of the oesophagus ; while all the 
movements of the animal are rhythmical to a degree not exem- 
plified in such simple forms as Amoeba; not to mention its 
various resources for multiplication and, therefore, for its 
perpetuation and permanence as a species. It, too, like all the 
unicellular organisms we have been considering, is susceptible 
of very wide distribution, being capable of retaining vitality in 
the dried state, so that these infusoria may be carried in vari- 
ous directions by winds in the form of microscopic dust. 


The Fresh-Water Polyps {Hydra viridis ; Hydra fusca). 

The comparison of an animal so simple in structure, though 
made up of many cells, as the Polyp, with the more complex 
organizations with which we shall have especially to deal, may 


be fitly undertaken at this stage. The Polyps are easily obtain- 
able from ponds in whicli they are found attached to various 
kinds of weeds. To the naked eye, they resemble translucent 
masses of jelly with a greenish or reddish tinge. They range 
in size from one quarter to one half an inch ; are of an elongated 
cylindrical form ; provided at the oral extremity with thread- 
like tentacles of considerable length, which are slowly moved 
about in all directions ; but they and the entire body may short- 
en rapidly into a globular mass. They are usually attached at 
the opposite (aboral) pole to some object, but may float free, or 
slowly crawl from place to place. It may be observed, under 
the microscope, that the tentacles now and then embrace some 
living object, convey it toward an opening (mouth) near their 
base, from which, from time to time, refuse material is cast out. 
It may be noticed, too, that a living object within the touch of 
these tentacles soon loses the power to struggle, which is owing 
to the peculiar cells {nettle-cells, wrticating capsules, nemato- 
cysts) with which they are abundantly provided, and which se- 
crete a poisonous fluid that paralyzes prey. 

The mouth leads into a simple cavity (cadom) in which 
digestion proceeds. The green color in Hydra viridis, and the 
red color of Hydra fusca, is owing to the presence of chlorophyl, 
the function of which is not known. Hydra is structurally a 
sac, made up of two layers of cells, an outer {ectoderm) and 
an inner {endoderm) ; the tentacles being repetitions of the 
structure of the main body of the animal, and so hollow and 
composed of two cell layers. Speaking generally, the outer 
layer is devoted to obtaining information of the surroundings ; 
the inner to the work of .preparing nutriment, and probably, 
also, discharging waste matters, in which latter assistance is 
also received from the outer layer. As digestion takes place 
largely within the cells themselves, or is intracellular, we are 
reminded of Vorticella and still more of Amoeba. There is in 
Hydra a general advance in development, but not very much in- 
dividual cell specialization. That of the urticating capsules is 
one of the best examples of such specialization in this creature. 
A Polyp is like a colony of Amoebae in which some division of 
labor (function) has taken place; a sort of biological state in 
which every individual is nearly equal to his neighbor, but 
somewhat more advanced than those neighbors not members of 
the organization. 

But in one respect the Polyps show an enormous advance. 
Ordinarily when nourishment is abundant hydra multiplies by 



Fig. 46. 


Figs. 41 to 46. — In the following figures, ec, denotes ectoderm ; en, endoderm ; f , tentacle ; Ap, 

hypostome ; /, foot ; ts, testes ; ot), ovary ; ps, pseudopodium ; ee\ larger ectoderm-cells ; 

ne\ larger nematoc^sts before rupture ; (^^ Kleinenberg^s fibers ; c.I, supporting lamella ; 

c/, chlorophyl-forming bodies ; c, cillum 
Fio. 41, — The green hydra, at the maximum of contraction and elongation of its body. The 

creature is represented in the act of seizing a small crustacean (A, S). 
Fig. 42.— Transverse section across the body of a hydra, in the digestive cavity of which a 

small crustacean is represented. 
Fig. 43. — The leading types of thread-cells, after liberation from the body (F, 3). The cells are 

represented in the active and the resting conditions ; in the former all the parts are more 

distinctly seen in consequence of the necessary eversion. 
Fig. 44.— Small portion of a transverse section across the body of a green hydra (D, 3). 
Fig. 45.— a large brown hydra bearing at the same time buds produced asexually and sexual 

Fig. 46.— Larger ceUs of the ectoderm isolated. Note the processes of the cells or EJeinen- 

berg's fibers. (F, 3.) 
All of the cuts on pages, 9, 11, 13, 16, 18, 21 and iJl.have been selected from Howes' "Atlas of 


budding, and wlien cut into portions each may become a com- 
plete individual. However, under other circumstances, near 
the bases of the tentacles the body wall may protrude into little 
masses {testes), in which cells of peculiar formation {sperma- 
tozoa) arise, and are eventually set free and unite with a cell 
{ovum) formed in a similar protrusion of larger size {ovary). 
Here, then, is the first instance in which distinctly sexual re- 
production has been met in our studies of the lower forms of 
life. This is substantially the same process in Hydra as in 
mammals. But, as both male and female cells are produced by 
the same individual, the sexes are united {hermaphroditism) ; 
each is at once male and female. 

Any one- watching the movements of a Polyp, and compar- 
ing it with those of a Bell-animalcule, will observe that the 
former are much less machine-like ; have greater range ; seem 
to be the result of a more deliberate choice ; are better adapted 
to the environment, and calculated to achieve higher ends. In 
the absence of a nervous system it is not easy to explain how 
one part moves in harmony with another, except by that process 
which seems to be of such wide application in nature, adapta- 
tion from habitual simultaneous effects on a protoplasm capable 
of responding to stimuli. When one process of an Amoeba is 
touched, it is likely to withdraw all. This we take to to be due 
to influences radiating through molecular movement to other 
parts ; the same principle of action may be extended to Hydra. 
The oftener any molecular movement is repeated, the more it 
tends to become organized into regularity, to become fixed in 
its mode of action ; and if we are not mistaken this is a funda- 
mental law throughout the entire world of living things, if not 
of all things animate and inanimate alike. To this law we 
shall return. 

But Hydra is a creature of but very limited specializations ; 
there are neither organs f f circulation, respiration, nor excretion. 


if we exclude the doubtful case of the thread-cells {urticating 
capsules). The animal breathes by the entire surface of the 
body ; nourishment passes from cell to cell, and waste is dis- 
charged into the water surrounding the creature from all cells, 
though probably not quite equally. All parts are not digestive, 
respiratory etc., to the same degree, and herein does it differ 
greatly from Amoeba or even Vorticella, though fuUei' knowl- 
edge will likely modify our views of the latter two and similar 
organisms in this regard. 


Having now studied certain one-celled plants and animals, 
and some very simple combinations of cells (molds, etc.), it will 
be profitable to endeavor to generalize the lessons these humble 
organisms convey ; for, as will be constantly seen in the study 
of the higher forms of life of which this work proposes to treat 
principally, the same laws operate as in the lowliest living 
creatures. The most complex organism is made up of tissues, 
which are but cells and their products, as houses are made of 
bricks, mortar, wood, and a few other materials, however large 
or elaborate. 

The student of physiology who proceeds scientifically must 
endeavor, in investigating the functions of each organ, to learn 
the exact behavior of each cell as determined by its own inher-. 
ent tendencies, and modified by the action of neighboring cells. 
The reason why the function of one organ differs from that of 
another is that its cells have departed in a special direction 
from those properties common to all cells, or have become func- 
tionally differentiated. But such a statement has no meaning 
unless it be well understood that cells have certain properties in 
common. This is one of the lessons imparted by the preceding 
studies which we now review. Briefly stated in language now 
extensively used in works on biology, the common properties of 
cells (protoplasm), whether animal or vegetable, whether con- 
stituting in themselves entire animals or plants, or forming the 
elements of tissues, are these : The collective chemical processes 
associated with the vital activities of cells are termed its meta- 
bolism. Metabolism is constructive when more complex com- 
pounds are formed from simpler ones, as when the Protococcus- 
cell builds up its protoplasm out of the simple materials, found 
in rain-water, which make up its food. Metabolism is destruct- 


ive when the reverse process takes place. The results of this 
process are eliminated as excreta, or useless and harmful prod- 
ucts. Since all the vital activities of cells can only be mani- 
fested when supplied with food, it follows that living organisms 
convert potential or possible energy into kinetic or actual en- 
ergy. When lifeless, immobile matter is taken in as food and, 
as a result, is converted by a process of assimilation into the 
protoplasm of the cell using it, we have an example of poten- 
tial being converted into actual energy, for one of the proper- 
ties of all protoplasm is its contractility. Assimilation implies, 
of course, the absorption of what is to be used, with rejection 
of waste matters. 

The movements of protoplasm of whatever kind, when due 
to a stimulus, are said to indicate irritability ; while, if inde- 
pendent of any external source of excitation, they are denomi- 
nated automatic. 

Among agents that modify the action of all kinds of proto- 
plasm are heat, moisture, electricity, light, and others in great 
variety, both chemical and mechanical. It can not be too well 
remembered that living things are what they are, neither by 
virtue of their own organization alone nor through the action 
of their environment alone (else would they be in no sense dif- 
ferent from inanimate things), but because of the relation of 
the organization to the surroundings. 

Protoplasm, then, is contractile, irritable, automatic, absorp- 
tive, secretory (and excretory), metabolic, and reproductive. 

But when it is affirmed that these are the fundamental prop- 
erties of all protoplasm, the idea is not to be conveyed that cells 
exhibiting these properties are identical biologically. No two 
masses of protoplasm can be quite alike, else would there be no 
distinction in physiological demeanor — no individuality. Every 
cell, could we but behold its inner molecular mechanism, differs 
from its neighbor. When this difference reaches a certain de- 
gree in one direction, we have a manifest differentiation leading 
to physiological division of labor, which may now with advan- 
tage be treated in the following section. 


An animal, as we have learned, may be made up of a single 
cell in which each part performs much the same work ; or, if 
there be differences in function, they are ill-defined as compared 


with those of higher animals. The condition of things in such 
an animal as Amoeba may he compared to a civilized commu- 
nity in a very crude social condition. When each individual 
tries to perform every office for himself, he is at once carpenter, 
blacksmith, shoemaker, and much more, with the natural re- 
sult that he is not efficient in any one direction. A community 
may be judged in regard to its degree of advancement by the 
amount of division of labor existing within it. Thus is it with 
the animal body. We iind in such a creature as the fresh-water 
Hydra, consisting. of two layers of cells forming a simple sac, a 
slight amount of advancement on Amoeba. ItSSfexternal surface 
' no longer serves for inclosure of food, but it has the simplest 
form of mouth and tentacles. Each of the cells of the internal 
layer seems to act as a somewhat improved or specialized Amoe- 
ba, while in those of the outer layer we mark a beginning of 
those functions which taken collectively give the higher ani- 
mals information of the surrounding world. 

Looking to the existing state of things in the universe, it is 
plain that an animal to attain to high ends must have powers 
of rapid locomotion, capacity to perceive what makes for its in- 
terest, and ability to utilize means to attain this when perceived. 
These considerations demand that an animal high in the scale 
. of being should be provided with limbs sufficiently rigid to sup- 
port its weight, moved by strong muscles, which must act in 
harmony. But this implies abundance of nutriment duly pre- 
pared and regularly conveyed to the bones and muscles. All 
this would be useless unless there was a controlling and ener- 
gizing system capable both of being impressed and originating 
impressions. Such is found in the nerves and nerve-centers. 
Again, in order that this mechanism be kept in good running 
order, the waste of its own metabolism, which chokes and poi- 
sons, must be got rid of — hence the need of excretory apparatus. 
In order that the nervous system may get sufficient informa- 
tion of the world around, the surface of the body must be pro- 
vided with special message-receiving offices in the form of 
modified nerve-endings. In short, it is seen that an animal as 
high in the scale as a mammal must have muscular, osseous 
(and connective)., digestive, circulatory, excretory, and nervous 
tissues ; and to these may be added certain forms of protective 
tissues, as hair, nails, etc. 

Assuming that the student has at least some general knowl- 
edge of the structure of these various tissues, we propose to tell 
in a simple way the whole physiological story in brief. 


The blood is tlie source of all the nourishment of the organ- 
ism, including its oxygen supply, and is carried to eveTy part of 
the body through elastic tubes which, continually branching 
and becoming gradually smaller, terminate in vessels of hair- 
like fineness in which the current is very slow — a condition per- 
mitting that interchange between the cells surrounding them 
and the blood which may be compared to a process of barter, 
the cells taking nutriment and oxygen, and giving (excreting) 
in return carbonic anhydride. From these minute vessels the 
blood is conveyed back toward the source whence it came by 
similar elastic tubes which gradually increase in size and be- 
come fewer. The force which directly propels the blood in its 
onward course is a muscular pump, with both a forcing and 
• suction action, though chiefly the former. The flow of blood 
is maintained constant owing to the resistance in the smaller 
tubes on the one hand and the elastic recoil of the larger tubes 
on the other ; while in the returning vessels the column of 
blood is supported by elastic double gates which so close as to 
prevent reflux. The oxygen of the blood is carried in disks of 
microscopic size which, give it up in proportion to the needs of 
the tissues past which they are carried. 

But in reality the tissues of the body are not nourished 
directly by the blood, but by a fluid derived from it and resem- 
bling it greatly in most particulars. This fluid bathes the 
tissue-cells on all sides. It also is taken up by tubes that 
convey it into the blood after it has passed through little fac- 
tories (lymphatic glands), in which it undergoes a regeneration. 
Since the tissues are impoverishing the blood by withdrawal of 
its constituents, and adding to it what is no longer useful, and 
is in reality poisonous, it becomes necessary that new material 
be added to it and the injurious components withdrawn. The 
former is accomplished by the absorption of the products of 
food digestion, and the addition of a rfresh supply of oxygen 
derived from without, while the poisonous ingredients that 
have found their way into the blood are got rid of through 
processes that may be, in general, compared to those of a sew- 
age system of a very elaborate character. To explain this re- 
generation of the blood in somewhat more detail, we must first 
consider the fate of food from the time it enters the mouth till 
it leaves the tract of the body in which its preparation is 
carried on. 

The food is in the mouth submitted to the action of a series 
of cutting and grinding organs worked by powerful muscles ; 


mixed with a fluid wliich changes the starchy part of it into 
sugar, and prepares the whole to pass further on its course: 
when this has been accomplished, the food is grasped and 
squeezed and pushed along the tube, owing to the action of its 
own muscular cells, into a sac (stomach), in which it is rolled 
about and mixed with certain fluids of peculiar chemical com- 
position derived from cells on its Inner surface, which trans- 
form the proteid part of the food into a form susceptible of 
ready use (absorption). When this saccular organ has done 
its share of the work, the food is moved on by the action of 
the muscles of its walls into a very long portion of the tract in 
which, in addition to processes carried on in the mouth and 
stomach, there are others which transform the food into a 
condition in which it can pass into the blood. Thus, all of 
the food that is susceptible of changes of the kind described is 
acted upon somewhere in the long tract devoted to this task. 
But there is usually a remnant of indigestible material which 
is finally evacuated. How is the prepared material conveyed 
into the blood ? In part, directly through the walls of the 
minutest blood-vessels distributed throughout the length of 
this tube ; and in part through special vessels with appropriate 
cells covering them which act as minute porters (villi). 

The impure blood is carried periodically to an extensive sur- 
face, usually much folded, and there exposed in the hair-like 
tubes referred to before, and thus parts with its excess of car- 
bon dioxide and takes up fresh oxygen. But all the functions 
described do not go on in a fixed and invariable manner, but 
are modified somewhat according to circumstances. The for- 
cing-pump of the circulatory system does not always beat 
equally fast; the smaller blood-vessels are not always of the 
same size, but admit more or less blood to an organ according 
to its needs. 

This is all accomplished in obedience to the commands car- 
ried from the brain and spinal cord along the nerves. All 
movements of the limbs and other parts are executed in obe- 
dience to its behests ; and in order that these may be in accord- 
ance with the best interests of each particular organ and the 
whole animal, the nervous centers, which may be compared to 
the chief officers of, say, a telegraph or railway system, are in 
constant receipt of information bj' messages carried onward 
along the nerves. The command issuing is always related to 
the information arriving. 

All those parts commonly known as sense-organs^^the eye. 


ear, nose, tongue, and the entire surface of the body — are faith- 
ful reporters of facts. They put the inner and outer worlds in 
communication, and without them all higher life at least must 
cease, for the organism, like a train directed by a conductor that 
disregards the danger-signals, must work its own destruction. 
Without going into further details, suffice it to say that the pro- 
cesses of the various cells are subordinated to the general good 
through the nervous system, and that susceptibility of proto- 
plasm to stimuli of a delicate kind which enables each cell to 
adapt to its surroundings, including the influence of remote as 
well as neighboring cells. Without this there could be no 
marked advance in organisms, no differentiation of a pro- 
nounced character, and so none of that physiological division 
of labor which will be inferred from our brief description of 
the functions of a mammal. The whole of physiology but 
illustrates this division of labor. 

It is hoped that the above account of the working of the 
animal body, brief as it is, may serve to show the connection of 
one part functionally with another, for it is much more impor- 
tant that this, should be kept in mind throughout, than that all 
the details of any one function should be known. 


In order to enable the student the better to realize the na- 
ture of living matter or protoplasm, and to render clearer 
the distinction between the forms that belong to the organic 
and inorganic worlds respectively, we shall make some com- 
parisons in detail which it is hoped may accomplish this ob- 

A modern watch that keeps correct time must be regarded 
as a wonderful object, a marvelous triumph of human skill. 
That it has aroused the awe of savages, and been mistaken for 
a living being, is not surprising. But, admirable as is the 
result attained by the mechanism of a watch, it is, after all, 
composed of but a few metals, etc., chiefly in fact of two, brass 
and steel ; these are, however, made up into a great number 
of different parts, so adapted to one another as to work in 
unison and accomplish the desired object of indicating the time 
of day. 

Now, however well constructed the watch may be, there are 
■ waste, wear and tear, which will manifest themselves more and 


more, until finally tlie machine becomes wortliless for the pur- 
pose of its construction. If this mechanism possessed the 
power of adapting from without foreign matter so as to con- 
struct it into steel and brass and arrange this just when re- 
quired, it would imitate a living organism ; but this it can not 
do, nor is its waste chemically different from its component 
metals ; it does not break up brass and steel into something 
wholly different. In one particular it does closely resemble 
living things, in that it gradually deteriorates ; but the degra- 
dation of a living cell is the consequence of an actual change 
in its component parts, commonly a fatty degeneration. The 
one is a real transformation, the other mere wear. 

Had the watch the power to give rise to a new one like itself 
by any process, especially a process of division of itself into two 
parts, we should have a parallel with living forms; but the 
watch can not even renew its own parts, much less give rise to 
a second mechanism like itself. Here, then, is a manifest dis- 
tinction between living and inanimate things. 

Suppose further that the watch was so constructed that, 
after the lapse of . a certain time, it underwent a change in its 
inner machinery and perhaps its outer form, so as to be scarcely 
recognizable as the same ; and that as a result, instead of indi- 
cating the hours and minutes of a time-reckoning adapted to 
the inhabitants of our globe, it indicated time in a wholly dif- 
ferent way ; that after a series of such transformations it fell to 
pieces — took the original form of the metals from which it 
was constructed — we should then have in this succession of 
events a parallel with the development, decline, and death of 
living organisms. 

In another particular our illustration of a watch may serve 
a useful purpose. Suppose a watch to exist, the works of which 
are so concealed as to be quite inaccessible to our vision, so that 
all we know of it is that it has a mechanism which when in 
action we can hear, and the result of which we perceive in the 
movements of the hands on the face ; we should then be in the 
exact position in reference to the watch that we now are as re- 
gards the molecular movements of protoplasm. On the latter 
the entire behavior of living matter depends ; yet it is abso- 
lutely hidden from us. 

We know, too, that variations must be produced in the 
mechanism of time-pieces by temperature, moisture, and other 
influences of the environment, resulting in altered action. The 
same, as will be shown in later chapters, occurs in protoplasm. 


This, too, is primarily a molecular effect. If the works of 
watches were beyond observation, we should not be able to state 
exactly how the variations observed in different kinds, or even 
different individuals of the same kind occurred, though these 
differences might be of the most marked character, such as any 
one could recognize. Here once more we refer the differences 
to the mechanism. So is it with living beings: the ultimate 
molecular mechanism is unknown to us. 

Could we but render these molecular movements visible to 
our eyes, we should have a revelation of far greater scientific 
importance than that unfolded by the recent researches into 
those living forms of extreme minuteness that swarm every- 
where as dust in a sunbeam, and, as will be learned later, are 
often the source of deadly disease. Like the movements of the 
watch, the activities of protoplasm are ceaseless. A watch that 
will not run is, as such, worthless — it is mere metal — has under- 
gone an immense degradation in the scale of values; so proto- 
plasm is no longer protoplasm when its peculiar molecular 
movements cease ; it is at once degraded to the rank of dead 

The student may observe that each of the four propositions, 
embodying the fundamental properties of living matter, stated 
in the preceding chapter, have been illustrated by the simile of 
a watch. Such an illustration is necessarily crude, but it helps 
one to realize the meaning of truths which gather force with 
each living form studied if regarded aright ; and it is upon the 
realization of truth that mental growth as well as practical 
efficiency depends. 


There are human beings so low in the scale as not to possess 
such general terms as tree, while they do employ names for dif- 
ferent kinds of trees. The use of such a word as " tree " im- 
plies generalization, or the abstraction of a set of qualities from 
the things in which they reside, and making them the basis for 
the grouping of a multitude of objects by which we are sur- 
rounded. Manifestly without such a process knowledge must 
be very limited, and the world without significance ; while in 
proportion as generalization may be safely widened, is our 
progress in the unification of knowledge toward which science 
is tending. But it also follows that without complete knowl- 


edge there can be no perfect classification of objects; hence, 
any classification must be regarded but as the temporary creed 
of science, to be modified with the extension of knowledge. As 
a matter of fact, this has been the history of all zoological and 
other systems of arrangement. ' The only purpose of grouping 
is to simplify and extend knowledge ; this being the case, it fol- 
lows that a method of grouping that accomplishes this has 
value, though the system may be artificial that is based on 
resemblances which, though real and constant, are associated 
with differences so numerous and radical that the total amount 
of likeness between objects thus grouped is often less than the 
difference. Such a system was that of Linnaeus, who classified- 
plants according to the number of stamens, etc., they bore. 

Seeing that animals' which resemble each other are of com- 
mon descent from some earlier form, to establish the line of de- 
scent is to determine in great part the classification. Much as^ 
sistance in this direction is derived from embryology, or the 
history of the development of the individual ( ontogeny) ; so 
that it may be said that the ontogeny indicates, though it does 
not actually determine, the line of descent {phytogeny) ; and it 
is owing to the importance of this truth that naturalists have 
in recent years given so much attention to comparative embry- 

It will be inferred that a natural system of classification must 
be based both on function and structure, though chiefly on the 
latter, since organs of very different origin may have a similar 
function ; or, to express this otherwise, homologous structures 
may not be analogous; and homology gives the better basis for 
classification. To illustrate, the wing of a bat and a bird are 
both homologous and analogous; the wing of a butterfly is 
analogous but not homologous with these ; manifestly, to clas- 
sify bats and birds together would be better than to put birds 
and insects in the same group, thus leaving other points of re- 
lationship out of consideration. 

The broadest possible division of the animal kingdom is into 
groups, including respectively one-celled and many-celled 
forms — i. e., into Protozoa and Metasoa. As the wider the 
grouping the less are differences considered, it follows that the 
more subdivided the groups the more complete is the informa- 
tion conveyed : thus, to say that a dog is a metazoan is to con- 
vey a certain amount of information ; that it is a vertebrate, 
more ; that it is a mammal, a good deal more, because each of 
the latter terms includes the former. 



i' Protozoa (amoeba, vorticella, etc.). 
j Coelenterata (sponges, jelly-fish, polyps, etc.). 
I Echinodermata (star-fish, sea-urchins, etc.). 
Inverte- J Vermes (worms), 
brata. I Arthropods (crabs, insects, spiders, etc.). 
Mollusca (oysters, snails, etc.). 
I Molluscoidea (moss-like animals). 
[ Tunicata (ascidians). 

r Pisces (fishes). 
Amphibia (frogs, menobranehus, etc.). 
Vertebrata. \ Eeptilia (snakes, turtles, etc.). 
I Aves (birds). 
[ Mammalia (domestic quadrupeds, etc.). 

The above classification (of Glaus) is, like all sucli arrange- 
' ments, but the expression of one out of many methods of view- 
ing the animal kingdom. 

For the details of classification and for the grounds of that 
we have presented, we refer the student to works on zoology ; 
but we advise those who are not familiar with this subject, 
when a technical term is used, to think of that animal belong- 
ing to the group in question with the structure of which they 
are best acquainted. 

Man's Place in the Animal Kingdom. 

It is no longer the custom with zoologists to place man in 
an entirely separate group by himself ; but he is classed with 
the primates, among which are also grouped the anthropoid 
apes (gorilla, chimpanzee, orang, and the gibbon), the monkeys 
of the Old and of the New World, and the lemurs. So great is 
the structural resemblance of man and the other primates that 
competent authorities declare that there is more difference be- 
tween the structure of the most widely separated members of 
the group than between certain of the anthropoid apes and man. 

The points of greatest resemblance between man and the 
anthropoid apes are the following : The same number of verte- 
brse ; the same general shape of the pelvis ; a brain distinguish- 
ing them from other mammals ; and posture, being bipeds. 

The distinctive characters are size, rather than form of the 
brain, that of man being more than twice as large ; a relatively 
larger cranial base, by which, together with the greater size of 
the jaws, the face becomes prominent ; the earlier closure of 
the sutures of the cranium, arresting the growth of the brain ; 
more developed canine teeth and difference in the order of 
eruption of the permanent teeth ; the more posterior position 
of the foramen magnum ; the relative length of the limbs to 


each other and the rest of the body ; minor differences in the 
hands and feet, especially the greater freedom and power of 
apposition of the great-toe. 

But the greatest distinction between man and even his 
closest allies among the apes is to be found in the development 
to an incomparably higher degree of his intellectual and moral 
nature, corresponding to the differences in weight and structure 
of the human brain, and associated with the use of spoken and 
written language ; so that the experience of previous genera- 
tions is not only registered in the organism (heredity), but in a 
form more quickly available (books, etc.). 

The greatest structural difference between the races of men- 
are ' referable to the cranium; but, since they all interbreed 
freely, they are to be considered varieties of one species. 


The term rhythm to most minds suggests music, poetry, or 
dancing, in all of which it forms an essential part so simple, 
pronounced, and uncomplicated as to be recognized by all with 

The regular division of music into bars, the recurrence of 
chords of the same notes at certain intervals, of forte and piano, 
seem to be demanded by the very nature of the human mind. 
The same applies to poetry. Even a child that can not under- 
stand the language used, or an adult listening to recitations in 
an unknown tongue, enjoys the flow and recurrences of the 
sounds. Dancing has in all ages met a want in human organi- 
zations, which is partly supplied in quieter moods by the regu- 
larity of the steps in walking and similar simple movements. 

But as rhythm runs through all the movements of animals, 
so is it also found in all literature and all art. Infinite variety 
wearies the mind, hence the fatigue felt by the sight-seer. Re- 
currence permits of repose, and gratifies an established taste or 
appetite. The mind delights in what it has once enjoyed, in 
repetition within limits. Repetition with variety is manifestly 
a condition of the growth and development of the mind. This 
seems to apply equally to the body, for every single function 
of each organism, however simple or complex it may be, exem- 
plifies this law of periodicity. The heart's action is rhythmical 
(beats) ; the blood flows in intermitting gushes from the central 
pump ; the to-and-fro movements of respiration are so regular 


that their cessation would arouse the attention of the least 
instructed; food is demanded at regular intervals; the juices 
of the digestive tract are poured out, not constantly but period- 
ically; the movements by which the food is urged along its 
path are markedly rhythmic; the chemical processes of the 
body wax and wane like the fires in a furnace, giving rise to 
regular augnjentations of the temperature of the body at fixed- 
hours of the day, with corresponding periods of greatest bodily 
activity and the reverse. 

This principle finds perfect illustration in the nervous sys- 
tem. The respiratory act of the higher animals is effected 
through muscular movements dependent on regular waves of 
excitation reaching them along the nerves from the central cells 
which regularly discharge their forces along these channels. 
Were not the movements of the body periodic or rhythmical, 
instead of that harmony which now prevails, every muscular 
act would be a convulsion, though even in the movements of 
the latter there is a highly compounded rhythm, as a noise is 
made up of a variety of musical notes. The senses are subject 
to the same law. The eye ceases to see and the ear to hear and 
the hand to feel if continuously stimulated ; and doubtless in 
all art this law is unconsciously recognized. That ceases to be 
art which fails to provide for the alternate repose and excita- 
tion of the senses. The eye will not tolerate continuously one 
color, the ear a single sound. Why is a breeze on a warm day 
so refreshing ? The answer is obvious. 

Looking to the world of animate nature as a whole, it is 
noticed that plants have their period of sprouting, flowering, 
seeding, and decline ; animals are born, pass through various 
stages to maturity, diminish in vigor, and die. These events 
make epochs in the life-history of each species ; the recurrence 
of which is so constant that the agricultural and other arrange- 
ments even of savages are planned accordingly. That the in- 
dividuals of each animal group have a definite period of dura- 
tion is another manifestation of the same law. 

Superficial observation sufiices to furnish facts which show 
that the same law of periodicity is being constantly exemplified 
in the world of inanimate things. The regular ebb and flow of 
the tides ; the rise and subsidence of rivers ; the storm and the 
calm ; summer and winter ; day and night — are all recurrent, 
none constant. 

Events apparently without any regularity, utterly beyond 
any law of recurrence, when sufficiently studied are found to 


fall under the same principle. Thus it took some time to learn 
that volcanic eruptions occurred with a very fair degree of 

In judging of this and all other rhythmical events it must 
be borne in mind that the time standard is for an irregularity 
that seems large, as in the instance just referred to, becomes 
small when considered in relation to the million^ of years of 
geological time ; while in the case of music a trifling irregu- 
larity, judged by fractions of a second, can not be tolerated 
by the musical organization — which is equivalent to saying 
that the interval of departure from exact regularity seems 

As most of the rhythms of the universe are compounded of 
several, it follows that they may seem, until closely studied, 
very far from regular recurrences. This may be observed in 
the interference in the regularity of the tides themselves, the 
daily changes of which are subject to an increase and decrease 
twice in each month, owing to the influence of the sun and moon 
being then either coincident or antagonistic. 

In the functions of plants and animals, rhythms must be- 
come very greatly compounded, doubtless often beyond recog- 

Among the best examples of rhythm in animals are daily 
sleep and winter sleep, or hibernation ; yet, amid sleep, dreams 
or recurrences of cerebral activity are common — ^that is, one 
rhythm (of activity) overlies another (of repose). In like man- 
ner many hibernating animals do not remain constantly in their 
dormant condition throughout the winter months, but have 
periods of wakefulness ; the active life recurs amid the life of 
functional repose. 

To return to the world of inanimate matter, we find that the 
crust of the earth itself is made of layers or strata the result of 
periods of elevation and depression, of denudation and deposi- 
tion, in recurring order. 

The same law is illustrated by the facts of the economic and 
other conditions of the social state of civilized men. Periods 
of depression alternate with periods of revival in commercial 

There are periods when many more marriages occur and 
many more children are born, corresponding with changes in 
the material conditions which influence men as well as other 

Finally, and of special interest to the medical student, are 


tlie laws of rhythm in disease. Certain fevers have their regu- 
lar periods of attack, as intermittent fever ; while all diseases 
have their periods of exacerbation, however invariable the 
symptoms may seem to be to the ordinary observer or even to 
the patient himself. 

Doubtless the fact that certain hereditary diseases do not 
appear in the offspring at once, but only at the age at which 
they were manifested in the parents, is owing to the same 

Let us now examine more thoroughly into the real nature of 
this rhythm which pervades the entire universe. 

If a bow be drawn across a violin-string on which some small 
pieces of paper have been placed, these will be seen to fly off ; 
and if the largest string be experimented iipon, it can be ob- 
served to be in rapid to-and-fro motion, known as vibration, 
which motion is perfectly regular, a definit;e number of move- 
ments occurring within a measured period of time; in other 
words the motion is rhythmical. In strings of the finest size 
the motion is not visible, but we judge of its existence because 
of the result, which is in each instance a sound. Sound is to us, 
however, an affection of the nerve of hearing and the brain, 
owing to the vibrations of the ear caused by similar vibra- 
tions of the violin-strings. The movements of the nerves and 
nerve-cells are invisible and molecular, and we seem to be 
justified in regarding molecular movements as constant and 
associated with all the properties of matter ivhether living or 

We see, then, that all things living and lifeless are in con- 
stant motion, visible or invisible ; there is no such thing in the 
universe as stable equilibrium. Change, ceaseless change, is 
written on all things; and, so far as we can judge, these 
changes, on the whole, tend to higher development. Neither 
rhythm, however, nor anything else, is perfect. Even the mo- 
tions of planets are subject to perturbations or irregularities 
in their periodicity. This subject is plainly boundless in its 
scope. We have introduced it at this stage to prepare for its 
study in detail in dealing with each function of the animal 
body. If we are correct as to the universality of the law of 
rhythm, its importance in biology deserves fuller recognition 
than it has yet received in works on physiology ; it will, ac- 
cordingly, be frequently referred to in the future chapters of 
this book. 



Every one must have observed in himself and others the 
tendency to fall into set -ways of doing certain things, in which 
■will and clear purpose do not come prominently into view. 
Further observation shows that the lower animals exhibit this 
tendency, so that, for example, the habits of the horse or the dog 
may be an amusing reflection of those of the master. Trees are 
seen to bend permanently in the direction toward which the 
prevailing winds blow. 

The violin that has experienced the vibrations aroused by 
some master's hand acquires a potential musical capability not 
possessed by an instrument equally good originally, but the 
molecular movements of which never received such an educa- 

It appears, then, that underlying what we call habit, there is 
some broad law not confined to living things ; indeed, the law of 
habit appears to bei closely related to the law of rhythm we 
have already noticed. Certain it is that it is inseparable from 
all biological phenomena, though most manifest in those organ- 
isms provided with a nervous system, and in that system itself. 
What we usually call habit, however expressed, has its physical 
correlation in the nervous system. We may refer to it in this 
connection later : but the subject has relations so numerous and 
fundamental that it seems eminently proper to introduce it at 
this early stage, forming as it does one of those corner-stones of 
the biological building on which the superstructure must rest. 

When we seek to come to a final explanation of habit in this 
case, as in most others, in which the fundamental is involved, 
we are soon brought against a wall over which we are unable 
to climb, and through which no light comes to our intellects. 

We must simply believe, as the result of observation, that it 
is a law of matter, in all the forms manifested to us, to assume 
accustomed modes of behavior, perhaps we may say molecular 
movement, in obedience to inherent tendencies. But, to recog- 
nize this, throws a fiood of light on what would be inexplicable, 
even in a minor degree. We can not explain gravitation in it- 
self ; but, assuming its universality, replaces chaos by order in 
our speculations on matter. 

Turning to living matter, we look for the origin of habit in 
the apparently universal principle that primary molecular 
movement in one direction renders that movement easier after- 


ward, and in proportion to the frequency of repetition ; wliicli 
is equivalent to saying that functional activity facilitates func- 
tional activity. Once accepting this as of universal application 
in biology, we have an explanation of the origin, the compara- 
tive rigidity, and the necessity of habit. There must be a phys- 
ical basis or correlative of all mental and moral habits, as well 
as those that may be manifested during sleep, and so purely in- 
dependent of the will and consciousness. We are brought, in 
fact, to the habits of cells in considering those organs, and that 
combination of structures which makes up the complex individ- 
ual mammal. It is further apparent that if the cell can trans- 
mit its nature as altered by its experiences at all, then habits 
must be hereditary, which is known to be the case. 

Instincts seem to be but crystallized habits, the inherited 
results of ages of functional activity in certain well-defined 

To a being with a highly developed moral nature like man, 
the law of habit is one of great, even fearful significance. We 
make to-day our to-morrow, and in the present we are deciding 
the future of others, as our present has been made for us in part 
by our ancestors. We shall not pursue the subject, which is of 
boundless extent, farther now, but these somewhat general 
statements will be amplified and applied in future chapters. 


It is a matter of common observation that animals originate 
from like kinds, and plants from forms resembling themselves ; 
while most carefully conducted experiments have failed to show 
that living matter can under any circumstances known to us 
arise from other than living matter. 

That in a former condition of the universe such may have 
been the case has not been disproved, and seems to be the logical 
outcome of the doctrine of evolution as applied to the universe 

By evolution is meant the derivation of more complex and 
differentiated forms of matter from simpler and more homogene- 
ous ones. When this theory is applied to organized or living 
forms, it is termed organic evolution. There are two views of 
the origin of life : the one, that each distinct group of plants 
and animals was independently created ; while by " creation " is 
simply meant that they came into being in a manner we know 


not how, in obedience to the will of a First Cause. The other 
view is denominated the theory of descent with modification, 
the theory of transmutation, organic evolution etc., which 
teaches that all the various forms of life have been derived 
from one or a few primordial forms in harmony with the recog- 
nized principles of heredity and variability. The most widely 
known and most favorably received exposition of this theory is 
that of Charles Darwin, so that his views will be first presented 
in the form of a hypothetical case. Assume that one of a group 
of living forms varies from its fellows in some particular, and 
mating with another that has similarly varied, leaves progeny 
inheriting this characteristic of the parents, that tends to be 
still further increased and rendered permanent by successive 
pairing with forms possessing this variation in form, color, or 
whatever it may be. We may suppose that the variations may 
be numerous, but are always small at the beginning. Since all 
animals and plants tend to multiply faster than the means of 
support, a competition for the means of subsistence arises, in 
which struggle the fittest, as judged by the circumstances, 
always is the most successful ; and if one must perish outright, 
it is the less fit. If any variation arises that is unfavorable in ' 
this contest, it will render the possessor a weaker competitor : 
hence it follows that only useful variations are preserved. The 
struggle for existence is, however, not alone for food, but for 
anything which may be an advantage to its possessor. One 
form of the contest is that which results from the rivalry of 
members of the same sex for the possession of the females ; and 
as the female chooses the strongest, most beautiful, most active, 
or the supreme in some respect, it follows that the best leave 
the greatest number of progeny. This has been termed sexual 

In determining what forms shall survive, the presence of 
other plants or animals is quite as important as the abun- 
dance of food and the physical conditions, often more so. To 
illustrate this by an example : Certain kinds of clover are fer- 
tilized by the visits of the bumble-bee alone ; the numbers of 
bees existing at any one place depends on the abundance of the 
field-mice which destroy the nests of these insects ; the numbers 
of mice will depend on the abundance of creatures that prey on 
the mice, as hawks and owls ; these, again, on the creatures that 
specially destroy them, as foxes, etc. ; and so on, the chain of 
connections becoming more and more lengthy. 

If a certain proportion of forms varying similarly were sep- 


arated by any great natural barrier, as a chain of lofty mount- 
ains or an intervening body of water of considerable extent, 
and so prevented from breeding with forms that did not vary, 
it is clear that there would be greater likelibood of their differ- 
ences being preserved and augmented up to the point of their 
greatest usefulness. 

We may now inquire whether such has actually been the 
course of events in nature. The evidence may be arranged 
under the following heads : 

1. Morphology. — Briefly, there is much that is common to 
entire large groups of animals ; so great, indeed, are the resem- 
blances throughout the whole animal kingdom, that herein is 
found the strongest argument of all for the doctrine of descent. 
To illustrate by a single instance — fishes, reptiles, birds, and 
mammals possess in common a vertebral column bearing the 
same relationship to other parts of the animal. It is because of 
resemblances of this kind, as well as by their differences, that 
naturalists are enabled to classify animals. 

2. Embryology. — In the stages through which animals pass 
in their development from the ovum to the adult, it is to be ob- 
served that the closer the resemblance of the mature organism 
in different groups, the more the embryos resemble one another. 
Up to a certain stage of development the similarity between 
groups of animals, widely separated in their post-embryonic 
life, is marked : thus the embryo of a reptile, a bird, and a mam- 
mal have much in common in their earlier stages. The embryo 
of the mammal passes through stages which represent condi- 
tions which are permanent in lower groups of animals, as for 
example that of the branchial arches, which are represented by 
the gills in fishes. It may be said that the developmental his- 
tory of the individual (ontogeny) is a brief recapitulation of 
the development of the species (phylogeny). Apart from the 
theory of descent, it does not seem possible to gather the true 
significance of such facts, which will become plainer after the 
study of the chapters on reproduction. 

3. Mimicry may be cited as an instance of useful adapta- 
tion. Thus, certain beetles resemble bees and wasps, which lat- 
ter are protected by stings. It is believed that such groups of 
beetles as these arose by a species of selection ; those escaping 
enemies which chanced to resemble dreaded insects most, so 
that birds which were accustomed to prey on beetles, yet feared 
bees, would likewise avoid the mimicking forms. 

4. Rudimentary Organs. — Organs which were once functional 



Fig. 47.— Shows the embryos of four mammals in the three con;? sponding stages : of a hog (H), 
calf (C), rabbit (R), and a man (M). The conditions of the three different stages of devel- 
opment, which the three cross-rows (I, II, III) represent, are setected to correspond as 
exactly as po^ible. The first, or upper cross-row, I, represents a very early stage, with 
gill-openings, and without limbs. The second (middle) cross-row, II, shows a somewhat 
later stage, with the first rudiments of limbs, while the gill-openings are yet retained. 
The third (lowest) cross-row, III, shows a still later stage, with the limbs more developed 
and the gill-openings lost. The membranes and appendages of the embryonic body (the 
amnion, yelk-sac, aflantois) are omitted. The whole twelve figures are slightly magnified, 
the upper ones more than the lower. To facilitate the comparison, they are all reduced 
to nearly the same size in the cuts. All the embryos are seen from the left side ; the head 
extremity is above, the tail extremity below ; the arched back turned to the right. The 
letters indicate the same parts in all the twelve figures, namely : v, fore-brain ; «, twixt- 
brain ; m, mid-brain ; A., hind-brain ; n, after-brain ; r, spinal marrow ; e, nose ; a, eye ; 
o, ear ; fc, gill-arches ; gr, heart ; w^ vertebral column ; /, fore-limbs ; 6, hind-limbs ; s, tail. 
(After Haeckel.) 


in a more ancient form, but serve no use in the creatures in 
whicli they are now found, have reached, it is thought, their 
rudimentary condition through long periods of comparative 
disuse, in many generations. Such are the, rudimentary mus- 
cles of the ears of man, or the undeveloped incisor teeth found 
in the upper jaw of ruminants. 

5. Geographical DiBtribution. — It can not be said that animals 
and plants are always found in the localities where they are 
best fitted to flourish. This has been well illustrated within 
the lifetime of the present generation, for the animals intro- 
duced into Australia have many of them so multiplied as to 
displace the forms native to that country. But, if we assume 
that migrations of animals and transmutations of species have 
taken place, this difficulty is in great part removed. 

6. Paleontology. — The rocks bear record to the former exist- 
ence of a succession of related forms ; and, though all the in- 
termediate links that probably existed have not been found, 
the apparent discrepancy can be explained by the nature of 
the circumstances under which fossil forms are preserved ; and 
the " imperfection of the geological record." 

It" is only in the sedimentary rocks arising from mud that 
fossils can be preserved, and those animals alone with hard 
parts are likely to leave a trace behind them ; while if these 
sedimentary rocks with their inclosed fossils should, owing to 
enormous pressure or heat be greatly changed (metamorphosed), 
all trace of fossils must disappear — so that the earliest forms 
of life, those that would most naturally, if preserved at all, be 
found in the most ancient rocks, are wanting, in consequence 
of the metamorphism which such formations have undergone. 
Moreover, our knowledge of the animal remains in the earth's 
crust is as yet very incomplete, though, the more it is explored, 
the more the evidence gathers force in favor of organic evolu- 
tion. But it must be remembered that those groups constitut- 
ing species are in geological time intermediate links. 

7. Fossil and Existing Species. — If the animals and plants now 
peopling the earth were entirely different from those that flour- 
ished in the past, the objections to the doctrine of descent would 
be greatly strengthened; but when it is found that there is in 
some cases a scarcely broken succession of forms, great force is 
added to the arguments by which we are led to infer the con- 
nection of all forms with one another. 

To illustrate by a single instance : the existing group of 
horses, with a single toe to each foot, was preceded in geological 



time in America by forms with a greater number of toes, the 
latter increasing according to the antiquity of the group. 

Fig. 48.— Bones of the feet of the different genera of EquicUB (after Marsh), a, foot of Oo- 
hippus (Eocene) : 6, foot of Aiichitherium (Lower Auocene) ; c, foot of Hipparion (Plio- 
cene) ; (j, foot of the recent genus Equus. 

These forms occur in succeeding geological formations. It is 
impossible to resist the conclusion that they are related gene- 
alogically (phylogenetically). 

8. Progression. — Inasmuch as any form of specialization that 
would give an animal or plant an advantage in the struggle for 
existence would be preserved, and as in most cases when the 
competing forms are numerous such would be the case, it is 
possible to understand how the organisms that have appeared 
have tended, on the whole, toward a most pronounced pro- 
gression in the scale of existence. This is well illustrated 
in the history of civilization. Barbarous tribes give way be- 
fore civilized man with the numberless subdivisions of labor 
he institutes in the social organism. It enables greater num- 
bers to flourish as the competition is not so keen as if activities 
could be exercised in a few directions only. 

9. Domesticated Animals. — Darwin studied our domestic ani- 
mals long and carefully, and drew many important conclusions 
from his researches. He was convinced that they had all been 
derived from a few wild representatives, in accordance with the 
principles of natural selection. Breeders have, both consciously 
and unconsciously, formed races of animals from stocks which 
the new groups have now supplanted ; while primitive man had 
tamed various species which he kept for food and to assist in 
the chase, or as beasts of burden. It is impossible to believe 
that all the different races of dogs have originated from dis- 
tinct wild stocks, for many of them have been formed within 
recent periods ; in fact, it is likely that to the jackal, wolf, and 



fox, must we look for tlie wild progenitors of our dogs. Dar- 
win concluded that, as man had only utilized the materials Na- 
ture provided in forming his races of domestic animals, he had 
availed himself of the variations that arose spontaneously, and 
increased and fixed them by breeding those possessing the same 
variation together, so the like had occurred without his aid in 
nature among wild forms. 

Evolutionists are divided as to the origin of man himself ; 
some, like Wallace, who are in accord with Darwin as to the 

« 3 

Fio. 49.— Skeleton of hand or fore-foot of six mammals. I, man ; U, dog ; m, pig ; IV, ox ; 
V, tapir : VI, horse, r, radius ; u. ulna ; a, scaphoid ; &, semi-lunar ; c, triquetrum (cunei- 
form) : <2, trapezium ; e, trapezoid : /, capitatum (unciform process) ; g^ hamatum (unci- 
form bone) ; p. pisiform ; 1, thumb ; 3, digit ; 3, middle finger ; 4. ring-finger ; 5, little 
finger. (After Gegenbaur.) 

origin of living forms in general, believe that the theory of 
natural selection does not suffice to account for the intellectual 
and moral nature of man. Wallace believes that man's body 
has been derived from lower forms, but that his higher nature 
is the result of some unknown law of accelerated development ; 
while Darwin, and those of his way of thinking, consider that 
man in his entire nature is but a grand development of powers 
existing in minor degree in the animals below him in the scale. 
Summary. — Every group of animals and plants tends to in- 
crease in numbers in a geometrical progression, and must, if 
unchecked, overrun the earth. Every variety of animals and 
plants imparts to its offspring a general resemblance to itself, 
but with minute variations from the original. The variations 
of offspring may be in any direction, and by accumulation 





Fig. 65.— a, chimpanzee ; b, gorUla ; c, orang ; d, negro. (Haeckel.) 


Fig. 56.— Head o£ a nose-ape (Sein- Fig. 57.— Head of Julia Pas- 

nopithecus naaicun) from Bor- trana. (From a photo- 

neo. (After Brehm.) graph by Hintze.) 

constitute fixed differences by which a new group is marked 
off. In the determination of the variations that persist, the law 
of survival of the fittest operates. 


As has been already noticed, protoplasm, in whatever form, 
after passing through certain stages in development, undergoes 
a decline, and finally dies and joins the world of unorganized 
matter ; so that the permanence of living things demands the 
constant formation of new individuals. Groups of animals 
and plants from time to time become extinct ; but the lifetime 
of the species is always long compared with that of the individ- 
ual. Reproduction by division seems to arise from an exigency 
of a nutritive kind, best exemplified in the simpler organisms. 
When the total mass becomes too great to be supported by 
absorption of pabulum from without by the surface of the 
body, division of the organism must take place, or death ensues. 
It appears to be a matter of indifference how this is accom- 
plished, whether by fission, endogenous division, or gemmation, 
so long as separate portions of protoplasm result, capable of 
leading an independent existence. The very undifferentiated 
character of these simple forms prepares us to understand how 
each fragment may go through the same cycle of changes as 
the parent form. In such cases, speaking generally, a million 
individuals tell the same biological story as one ; yet these 
must exist as individuals, if at all, and not in one great united 
mass. But in the case of conjugation, which takes place some- 
times in the same groups as also multiply by division in its 
various forms, there is plainly an entirely new aspect of the 


case presented. We have already shown that no two cells, how- 
ever much alike they may seem as regards form and the cir- 
cumstances under which they exist, can have, in the nature of 
the case, precisely the same history, or be the subjects of ex- 
actly the same experiences. We have also pointed out that all 
these phenomena of cell-life are known to us only as adaptations 
of internal to external conditions ; for, though we may not be 
always able to trace this connection, the inference is justi- 
fiable, because there are no facts known to us that contradict 
such an assumption, while those that are within our knowledge 
bear out the generalization. We have already learned that liv- 
ing things are in a state of constant change, as indeed are all 
things ; we have observed a constant relation between certain 
changes in the environment, or sum total of the surrounding 
conditions, as, for example, temperature and the behavior of 
the protoplasm of plants and animals ; so that we must believe 
that any one form of protoplasm, however like another it may- 
seem to our comparatively imperfect observation, is different 
in some respects from every other — as different, relatively, as 
two human beings living in the same community during the 
whole of their lives ; and in many cases as unlike as individuals 
of very different nationality and history. We are aware that 
when two such persons meet, provided the unlikeness is not so 
great as to prevent social intercourse, intercommunication may 
prove very instructive. Indeed, the latter grows out of the 
former ; our illustration is itself explained by the law we are 
endeavoring to make plain. It would appear, then, that con- 
tinuous division of protoplasm without external aid is not pos- 
sible ; but that the vigor necessary for this must in some way 
be imparted by a particle (cell) of similar, yet not wholly like, 
protoplasm. This seems to furnish an explanation of the neces- 
sity for the conjugation of living forms, and the differentiation 
of sex. Very frequently conjugation in the lowest animals and 
plants is followed by long periods when division is the prevail- 
ing method of reproduction. It is worthy of note, too, that 
when living forms conjugate, they both become quiescent for a 
longer or shorter time. It is as though a period of preparation 
preceded one of extraordinary activity. We can at present 
trace only a few of the steps in this rejuvenation of life-stuff. 
Some of these have been already indicated, which, with others, 
will now be further studied in this division of our subject, both 
because reproduction throws so much light oU cell-life, and be- 
cause it is so important for the understanding of the physio- 


logical behavior of tissues and organs. It may be said to be 
quite as important tliat the ancestral history of the cells of an 
organism be known as the history of the units composing a 
community. A, B, and C can be much better understood if 
we know something alike of the history of their race, their an- 
cestors, and their own past ; so is it with the study of any indi- 
vidual, animal, or group of animals or plants. Accordingly, 
embryology, or the history of the origin and development of 
tissues and organs, will occupy a prominent place in the va- 
rious chapters of this work. The student will, therefore, at 
the outset be furnished with a general account of the subject, 
while many details and applications of principles will be left 
for the chapters that treat of the functions of the various organs 
of animals. The more knowledge the student possesses of zo- 
ology the better, while this science will appear in a new light 
under the study of embryology. 

Animals are divisible, according to general structure, into 
Protozoa, or unicellular animals, and Metazoa, or multicellular 
forms — that is, animals composed of cell aggregates, tissues, or 
organs. Among the latter one form of reproduction appears 
for the first time in the animal kingdom, and becomes all but 
universal, though it is not the exclusive method ; for, as seen in 
Hydra, both this form of generation and the more primitive 
gemmation occur. It is known as sexual multiplication, which 
usually, though not invariably, involves conjugation of two un- 
like cells which may arise in the same or different individttals. 
That these cells, known as the male and female elements, the 
ovum and the spermatozoon, are not necessarily radically differ- 
ent, is clear from the fact that they may arise in the one individ- 
ual from the same tissue and be mingled together. These cells, 
however, like all others, tell a story of continual progressive 
differentiation corresponding to the advancing evolution of 
higher from lower forms. Thus hermaphroditism, or the coex- 
istence of organs for the production of male and of female cells 
in the same individual, is confined to invertebrates, among 
which it is rather the exception than the rule. Moreover, in 
such hermaphrodite forms the union of cells with greater differ- 
ence in experiences is provided for by the union of different in- 
dividuals, so that commonly • the male cell of one individual 
unites with (fertilizes) the female cell of a different individual. 
It sometimes happens that among the invertebrates the cells 
produced in the female organs of generation possess the power 
of division, and continued development wholly independently of 


the access of any male cell (parthenogenesis) ; sucli, however, is 
almost never the exclusive method of increase for any group of 
animals, and is to be regarded as a retention of a more ancient 
method, or perhaps rather a reversion to a past biological con- 
dition. No instance of complete parthenogenesis is known 
among vertebrates, although in birds partial development of the 
egg may take place independently of the influence of the male 
sex. The best examples of parthenogenesis are to be found 
among insects and crustaceans. 

It is to be remembered that, while the cells which form the 
tissues of the body of an animal have become specialised to 
discharge one particular function, they have not wholly lost 
all others ; they do not remain characteristic amoeboids, as we 
may term cells closely resembling Amoeba in behavior, nor do 
they wholly forsake their ancestral habits. They all retain the 
power of reproduction by division, especially when young and 
most vigorous ; for tissues grow chiefly by the production of 
new cells rather than the enlargement of already mature ones. 
Cells wear out and must be replaced, which is effected by the 
processes already described for Amoeba and similar forms. 
Moreover, there is retained in the blood of animals an army of 
cells, true amoeboids, ever ready to hasten to repair tissues lost 
by injury. These are true remnants of an embryonic condition ; 
for at one period all the cells of the organism were of this 
undifferentiated, plastic character. But the cell (ovum) from 
which the individual in its entirety and with all its complexity 
arises mostly by the union with another cell {spermatozoon), 
must be considered as one that has remained unspecialized 
and retained, and perhaps increased its reproductive functions. 
They certainly have become more complex. The germ-cell 
may be considered unspecialized as regards other functions, but 
highly specialized in the one direction of exceedingly great 
capacity for growth and complex division, if we take into ac- 
count the whole chain of results ; though in considering this it 
must be borne in mind that after a certain stage of division 
each individual cell repeats its ancestral history again ; that is 
to say, it divides and gives rise to cells which progress in turn 
as well as multiply. From another point of view the ovum is 
a marvelous storehouse of energy, latent or potential, of course, 
but under proper conditions liberated in varied and unexpected 
forms of force. It is a sort of storehouse of biological energy 
in the most concentrated form, the liberation of which in sim- 
pler forms gives rise to that complicated chain of events which 



is termed by the Liologist development, but wMch may be ex- 
pressed by the physiologist as the transformation of potential 
into kinetic energy, or the energy of motion. Viewed chemic- 
ally, it is the oft-repeated story of the production of forms, of 
greater stability and simplicity, from more unstable and com- 
plex ones, involving throughout the process of oxidation ; for it 
must ever be kept in mind that life and oxidation are concomi- 
tant and inseparable. The further study of reproduction in the 
concrete will render the meaning and force of many of the 
above statements clearer. 

The Ovum. 

The typical female cell, or ovum, consists of a mass of proto- 
plasm, usually globular in form, containing a nucleus and nu- 

The ovum may or may not be invested by a membrane ; the 
protoplasm of the body of the cell is usually highly granular, 
and may have stored up within it a varying amount of proteid 
material {food-yelk), which has led to division of ova into 
classes, according to the manner of distribution of this nutri- 
tive reserve. It is either concentrated at one pole {telolecith- 
al) ; toward the center {centrolecithal) ; or evenly distributed 

throughout {alecithdl). Dur- 
ing development this material 
is converted by the agency of 
the cells of the young organ- 
ism {embryo) into active pro- 
toplasm ; . in a word, they feed 
upon and assimilate or build 
up this food-stuff into their 
own substance, as Amoeba does 
with any proteid material it 

The nucleus {germinal vesi- 
cle) is large and well-defined, 
and contains within itself a 
highly refractive nucleolus 
{germinal spot). These closely 
resemble in general the rest of 
the cell, but stain more deeply and are chemically different in 
that they contain nucleine {nucleoplasm, chromatin). 

It will be observed that the ovum differs in no essential par- 

FiG. 58. — Semi-diagrammatic representation 
of a mammalian ovum (Schafer). Highly 
magnified, zp, zona pellucida ; m, vitel- 
lus ; firu, germinal vesicle ; gs^ germinal 






a ** ■ - - .. . , 

Fig. 59, — A human egg (much enlarged) from the ovary of a female. The whole egg is a 
simple spherical cell. The greater part of this cell is formed by the egg-yelk, by the gran- 
ular celt-substance (protoplasm), consisting of innumerable yelk-granules with a little 
inter-granular substance. In the upper part of the yelk lies the bright, globular, germ- 
vesicle, corresponding with the cell-kernel (mtcleus). This contains a darker germ-spot, 
answering to the nucleolus. The globular yelk-mass is surrounded by a thick, light- 
colored egg-membrane (zona pellucida^ or chorion). This is traversed by very numerous 
hair-like Imes, radiating toward the central point of the mass ; these are the porous 
canals, through which, in the course of fertilization, the thread-shaped, active sperm-cells 
penetrate into the egg-yelk. (Haeckel.) 

ticular of structure from other cells. Its difPerences are hidden 
ones of molecular structure and functional behavior. In ac- 
cordance with the diverse circumstances under which ova 
mature and develop, certain variations in structure, mostly of 
the nature of additions, present themselves. 

Thus, ova may be naked, or provided with one or more 
coverings. In vertebrates there are usually two membranes 
around the protoplasm of the ovum : a delicate covering ( Vi- 
telline membrane), beneath which there is another, which 
is sieve-like from numerous perforations {zona radiata, or s. 
pellucida). The egg membrane may be impregnated with lime 
salts (shell). Between the membranes and the yelk there is a 
fluid albuminous substance secreted by the glands of the ovi- 
duct, or by other special glands, which provide proteid nutri- 
ment in different physical condition from that of the yolk. 

The general naked-eye appearances of the ovum may be 
learned from the examination of a hen's egg, which is one of 




tlie most complicated known, inasmuch as it is adapted for . 
development outside of the body of the mother, and must, con- 
sequently, he capable of preserving its form and essential vital 
properties in a medium in which it is liable to undergo loss of 
water, protected as it now is with shell, etc., but which, at the 


Fig. (30.~Diag:raramatic section of an unimpreprnated fowl's egg: (Foster and Balfour, after 
Allen Thomson). 6/, blastoderm or cicatricula ; w. ?/, white yblk ; y. ?/, yellow yelk ; cli. I, 
ehalaza ; i. s. m, inner layer of shell membrane ; s. ?n, outer layer of shell niemhrane ; s, 
shell ; o. c. h, air-space ; ^y, the white of the eff^ ; v. t, vitelUne membrane ; x, the denser 
albuminous layer lying next the vitelline membrane. 

same time, permits the entrance of oxygen and moisture, and 
conducts heat, all being essential for the development of the 
germ within this large food-mass. The shell serves, evidently, 
chiefly for protection, since the eggs of serpents (snakes, turtles, 
etc.) are provided only with a very toiigh membranous cover- 
ing, this answering every purpose in eggs buried in sand or 
otherwise protected as theirs usually are. As the hen's egg is 
that most readily studied and most familiar, it may be well to 
describe it in somewhat further detail, as illustrated in the 
above figure, from the examination of which it will be ap- 
X^arent that the yelk itself is made up of a white and yellow 
portion distributed in alternating zones, and composed "of cells 
of different microscopical appearances. The clear albumen is 

The relative distribution, and the nature of the accessory or 
non-essential parts of the hen's egg, will be understood when it 
is remembered that, after leaving its seat of origin, which will 
be i^resently described, the ovum passes along a tube (oviduct) 



by a movement imparted to it by the muscular walls of tbe 
latter, similar to that of the gullet during the swallowing of 
food ; that this tube is provided with glands which secrete in 
turn the albumen, the membrane (outer), the lime salts of the 
shell, etc. "• The twisted appearance of the rope-like structures 
(chalazce) at each end is owing to the spiral rotatory movement 
the egg has undergone in its descent. 

The air-chamber at the larger end is not present from the 
first, but results from evaporation of the fluids of the albumen 
and the entrance of atmospheric air after the egg is laid some 

The Origin and Development op the Ovum. 

Between that protrusion of cells which gives rise to the 
bud which develops directly into the new individual^ and that 
which forms the ovary with- 
in which the ovum as a mod- 
ified cell arises, there is not 
in Hydra much difference at 
first to be observed. 

In the mammal, however, 
the ovary is a more complex 
structure, though, relatively 
to many organs, still simple. 
It consists, in the main, of 
connective tissue supplied 
with vessels and nerves in- 
closing modifications of that 
tissue {Graafian follicles) 
within which the ovum is 
matured. The ovum and the 
follicles arise from an inver- 
sion of epithelial cells, on a 
portion of the body cavity 
{germinal ridge), which give 
rise to the ovum itself, and 
the other cells surrounding 
it in the Graafian follicle. 
At first these inversions form 
tubules {egg-tubes) which lat- 
er become broken up into iso- 
lated nests of cells, the fore-runners of the Graafian follicles. 

The Graafian follicle consists externally of a fibrous capsule 

Fig. 61 

■Section through portion of the ovary 
of mammal, illustrating mode of develop- 
ment of the Graafian follicles (Wieder- 
sheimV D, discus prohgerus ; Ei\ ripe ovum ; 
G, follicular cells of germinal epithelium ; 
g, blood-vessels : 7C germinal vesicle (nucle- 
us) and germinal spot (nucleolus) : KE^ ger- 
minal epithelium ; i/, liquor folhculi ; Jtfg, 
membrana or tunica granulosa, or follicular 
epithelium ; Mp^ zona pellucida ; PS, in- 
growths from the germinal epithelium, ova- 
rian tubes, by means of which some of the 
nests retain their connection with the epithe- 
lium ; S, cavity which appears within the 
Graafian follicle ; So, stroma of ovary ; Tf, 
theoa f oUiculi or capsule ; V, primitive ova. 
When an ovum with its surrounding cells 
has become separated from the nest, it is 
known as a Gr^iian follicle. 



{tunica fibrosa), in close relation to -which is a layer of capillary 
blood-vessels {tunica vasculosa), the two together forming the 

Fig. 62.— Sagittal section of the ovary of an adult bitcli (after Waldeyer). o. e, ovarian epi- 
thelium ; o. <, ovarian tubes ; y.f, younger follicles ; o./, older follicle ; d. p, discus pro- 
ligerus, with the ovum ; e, epithelium of a second ovum in the same follicle ; /. c, fibrous 
coat of the follicle ; p. c, proper coat of the follicle ; e. /, epithelium of the f oUicle (mem- 
brana granulosa) ; a. /, collapsed atrophied follicle ; b. v, blood-vessels ; c. (, cell-tubes of 
the parovarium, divided longitudinally and transversely ; t. d, tubular depression of the 
ovarian epithelium, in the tissue of the ovary ; b. e, beginning of the ovarian epithelium, 
close to the lower border of the ovary. 

general covering {tunica propria) for the more delicate and im- 
portant cells within. Lining the tunic is a layer of small, some- 
what cubical cells {membrana granulosa), which at one part 
invest the ovum several layers deep {discus proligerus), while 
the remainder of the space is filled by a fluid {liquor folliculi) 
probably either secreted by the cells themselves, or resulting 
from the disintegration of some of them, or both. 



In viewing a section of the ovary taken from a mammal at 
the breeding-season, ova and Graafian follicles may be seen in 
all stages of development — those, as a rule, nearest the surface 
being the least matured. The Graafian follicle appears to pass 
inward, to undergo growth and development and again retire 
toward the exterior, where it bursts, freeing the ovum, which is 
conducted to the site of its future development by appropriate 
mechanism to be described hereafter. 

Changes in the Ovum itself. — The series of transformations 
that take place in the ovum before and immediately after the 
access of the male element is, in the opinion of many biolo- 
gists, of the highest significance, as indicating the course evolu- 
tion has followed in the animal kingdom, as well as instructive 
in illustrating the behavior of nuclei generally. 

The germinal vesicle may acquire powers of slow movement 
(amoeboid), and the germinal spot disappear : the former passes 
to one surface {pole) of the ovum ; both these structures may 
undergo that peculiar form of rearrangement {karyokinesis) 
which may occur in the nuclei and nucleoli of other cells prior 
to division ; in other words, the ovum has features common to 
it and many other cells in that early stage which precedes the 
complicated transformations which constitute the future his- 
tory of the ovum. 

A portion of the changed nucleus {aster) with some of the 
protoplasm of the cell accumulates at one surface {pole), which 

Via. 63.— Formation of polar cells in a star-fish (Asterias glacialis) (from Geddes, A— K after 
Fol, L after O. Hertwie:). A, ripe ovum with eccentric germinal vesicle and spot ; B — D, 
gradual metamorphosis of germinal vesicle and spot, as seen in the living egg, into two 
asters ; F, formation of first polar cells and withdrawal of remainmg part of nuclear 
spindle within the ovum ; G, surface view of hving ovum in the first polar cell ; H, com- 
pletion of second polar cell ; I, a later stage, showing the remaining internal half of the 
spindle in the form of two clear vesicles ; K, ovum with two polar cells and radial strife 
round female pronucleus, as seen in the living egg (E, F, H, and I from picric acid prepa- 
rations) ; L, expulsion of the first polar cell. (Haddon.) 

is termed the upper pole because it is at this region that the epi- 
thelial cells will be ultimately developed, and is separated ; this 
process is repeated. These bodies {polar cells, polar globules, 



etc.), then, are simply expelled ; they take no part in the devel- 
opment of the ovum ; and their extrusion is to be regarded as a 
preparation for the progress of the cell, whether this event fol- 
lows or precedes the entrance of the male cell into the ovum. 
It is worthy of note that the ovum may become amoeboid in the 
region from which the polar globules are expelled. 

The remainder of the nucleus {female pronucleus) now 
passes inward to undergo further changes of undoubted im- 
portancei possibly those by virtue of which all the subsequent 
evolution of the ovum is determined. This brings us to the 
consideration of another cell destined to play a brief but im- 
portant role on the biological stage. 

■ The Male Cell {Spermatozoon). 

This cell, almost without exception, consists of a nucleus 
(head) and vibratile cilium. However, as indicating that the 

Fig. 64.— Spennatozoa (after Haddon). Not drawn to scale. 1, sponge ; 2, hydroid ; .3, nema- 
tode ; 4, cray-flsh ; 5. snail ; 6, electric ray ; 7, salamander ; 8, horse ; 9, man. In many 
spermatozoa, as in Nos. 7 and 9, an extremely delicate vibratile band is present. 



latter is not essential, spermatozoa without such an appendage 
do occur. The obvious purpose of the cilium is to convey the 
male cell to the ovum through a fluid medium — either the water 
in which the ova are discharged in the case of most inverte- 
brates, or through the fluids that overspread the surfaces of the 
female generative organs. 

The Origin of the Spermatozoon. — The structures devoted to 
the production of male cells {testes), when reduced to their es- 

FiG. 65. — SpermatogeneEiS. A— H, isolated sperm-cells of the rat, showing the development 
of the spermatozoon and the gradual transformation of the nucleus into the spermatozoSu 
head. In G the seminal granule is being cast off (after H. H. Brown). I— M, sperm-cells 
of an Elasmobranch. The nucleus of each cell divides into a large number of daughter- 
nuclei, each one of which is converted into the rod-like head of a spermatozoon. N, trans- 
verse section of a ripe cell, showing the bundle of spermatozoa and the passive nucleus 
(I— N, after Semper). O — S, spermatogenesis in the earth-worm : O, young sperm-cell ; 
P, the same divided into fom- ; Q, spermatosphere with the central sperm- blastophore ; 
R, a later stage ; S, nearly mature spermatozoa. (After Blomfield.) 

sentials, consist of tubules, of great length in mammals, lined 
with nucleated epithelial cells, from which, by a series of 


changes figured above, a general idea of their development may- 
be obtained. 

It will be observed that throughout the series the nucleus of 
the cell is in every case preserved, and finally becomes the head 
of the male cell. Once more we are led to see the importance 
of this structure in the life of the cell. 

Fertilization of the Otiuu. — The spermatozoon, lashing its way 
along, when it meets the ovum, enters it either through a special 
minute gateway (micropyle), or if this be not present — as it is 
not in the ova of all anima-ls — it actually penetrates the mem- 
branes and substance of the female cell, and continues active 
till the female pronucleus is reached, when the head enters and 
the tail is absorbed or blends with the female cell. The nucleus 
of the male cell prior to union with the nucleus of the ovum 
undergoes changes similar to those that the nucleus of the 
ovum underwent, and thus becomes fitted for its special func- 
tions as a fertilizer ; or perhaps it woul^ be more correct to say 
that these altered masses of nuclear substance mutually fertil- 
ize each other, or initiate changes the one in the other which 
conjointly result in the subsequent stages of the development 
of the ovum. The altered male nucleus {male pronucleus), on 
reaching the female pronucleus, finds it somewhat ameeboid, 
a condition which may be shared in some degree by the entire 


Fib; 66.— Fertilization of ovum of a mollusk {Elysia viridis). A. Ovum sending up a protu- 
berance to meet the spermatozoSn, B. Approach of male pronucleus to meet me female 
pronucleus. F. FN, female pronucleus ; M. PN, male pronucleus ; S, spermatozoon. 

ovum. The resulting union gives rise to the new nucleus {seg- 
mentation nucleus), which is to control the future destinies of 
the cell ; while the cell itself, the fertilized ovum {oosperm), 
enters upon new and marvelous changes. 

In reality this process was foreshadowed in the dim past of 
the history of living things by the conjugation of infusoria 
and kindred animal and vegetable forms. When lower forms 
(unicellular) conjugate they become somewhat amoeboid sooner 


or later, and division of cell contents results. In some cases 
(septic monads) the resulting cell may burst and give rise to a 
shower of animal dust visible only by the highest powers of the 
microscope, each particle of which proves to be the nucleus 
from which a future individual arises. 

The study of reproduction thus establishes the conception of 
a unity of method throughout the animal and, it may be added, . 
the vegetable kingdom, for reproduction in plants is in all main 
points parallel to that process in animals. 

But why that costly loss of protoplasm by polar globules ? 
For the present we shall only say that it appears necessary to 
prevent parthenogenesis ; or at least to balance the share which 
the male and female elements take in the work of producing a 
new creature. It is to be remembered that both the male and 
female lose much in the process — blood, nervous energy, etc., in 
the case of the female, while the male furnishes a thousand-fold 
more cells than are used. But the period when organisms are 
best fitted for reproduction is that during which they are also 
most vigorous, and can best afford the superfluous drain on 
their energies. 

Segmentation and Subsequent Changes. 

After the changes described in the last chapter a new epoch 
in the biological history of the ovum — ^now the oosperm (or fer- 
tilized egg) — begins. A very distinct nucleus {segmentation 
nucleus) again appears, and the cell assumes a circular outline. 
The segmentation or division of the ovum into usually fairly 
equal parts now commences. This process can be best watched 
in the microscopic transparent ova of aquatic animals which 
undergo perfect development up to a certain advanced stage 
in the ordinary water of the ocean, river, lake, etc., in which 
the adult lives. 

Segmentation among invertebrates will be first studied, and 
for this purpose an ovum in which the changes are of a direct 
and uncomplicated nature will be chosen. 

The following figures and descriptions apply to a moUusk 
{Elysia viridis). We distinguish in ova resting stages and stages 
of activity. It is not, however, to be supposed that absolute 
rest ever characterizes any living form, or that nothing is tran- 
spiring because all seems quiet in these little biological worlds ; 
for we have already seen reason for believing that'life and in- 
cessant molecular activity are inseparable. It may be that, in 



the case of resting ova, changes of a more active character than 
usual are going on in their molecular constitution ; but, on the 
other hand, there may be really a diminution of these activities 
in correspondence with the law of rhythm. This seems the 
more probable. The meaning, however, of a "resting stage " is 

Fig. 67.— Primitive eggs of various animals, performing amoeboid movements (very much 
enlarged). All primitive eggs are naked cells, capable of change of form. Within the 
dark, finely granulated protoplasm (egg-yelk) lies a large vesicular kernel (the germ- 
vesiclej, and m the latter is a nucleolus (germ-spot); in the nucleolus a germ-point (nucleo- 
linus) is often visible. Fig. A 1-~A 4. The primitive egg of a chalk sponge {Leuculmis 
echinus), in four consecutive conditions of motion. Fig. B 1—B 8. The primitive egg of a 
hermit-crab (Chondracanthus corrmtus), in eight consecutive conditions of motion (after 
E. Van Beneden). Fig. C 1— C 5. Primitive egg of a cat, in four different conditions of 
motion (after PflOger). Fig. D. Primitive egg of a trout. Fig. E. Primitive egg of a hen. 
Fig. F. Primitive human egg. (Haeckel.) 

the obvious one of apparent quiescence— cessation of all kinds 
of movement. Then ensues rapidly and in succession the fol- 
lowing series of transformations : The nucleolus divides, later 



the nucleus, into two parts. These new nuclei then wander 
away from each other in opposite directions, and, losing their 

FiQ. 68.— Early stages of segmentation of a moUusk, Elysia viridls (drawn from the living 
egg). A, oosperm in state of rest after the extrusion of the polar cells : B, the nucleolus 
alone has divided ; C, the nucleus is dividing ; D, the nucleus, as such, has disappeared, 
first segmentation furrow appears ; E, later stage ; F, oosperm divided into two distinct 
segmentation spheres, the clear nuclear space m the center of the aster of granules is 
growing larger ; G, resting stage of appressed two spheres ; H, I, similar stages in the 
production of four spheres ; E, formation of eight-celled stage. (Haddon.) 

character as nuclei and nucleoli, are replaced by asters {polar 
stars), which seem to arise in the protoplasm of the body of 
the cell, and which are in close juxtaposition at first, but later 
separate, the oosperm becoming amoeboid in one region at 
least. A groove, which gradually deepens, appears on the sur- 
face, and finally divides the cell into two halves, which at once 
become flattened against each other. The nucleus may again 
be recognized in the center of each polar star, while a new nu- 
cleolus also reappears within the nucleus, when again a brief 
period of rest ensues. In the division and reformation of the 
nucleus, when most complicated (karyoMnesis), the changes 
may be generalized as consisting of division and segregation, 
followed by aggregation. 

The subdivision {segmentation) of the cell, after the quies- 
cence referred to, again commences, but in a plane at right 
angles to the first, from which four spheres result, again to be 
followed by the resting stage. The process continues in the 
same way, so that there is a progressive increase in the num- 
ber of segments, at least up to the point when a large number 



has been formed. This is rather to be considered as a type of 
one form of segmentation than as applicable to all, for even 
at this early stage differences are to be noted in the mode of 
segmentation which characterize effectually certain groups of 
animals ; but in all there is segmentation, and that segmenta- 
tion is rhythmical. 

Fig. 69.— The cleavage of a frog's egg (10 times enlarged). A, the parent-cell ; B, the two 
first cleavage-cells ; C, 4 cells ; D, 8 cells (4 animal and 4 vegetative) ; S, 12 cells (8 animal 
and 4 vegetative') ; f\ 16 cells (8 animal and 8 vegetative) ; (?, S4 cells (16 animal and 8 
vegetative) ; H, .32 cells ; /, 48 cells ; K, 64 cells ; X, 96 cleavage-cells ; M, 160 cleavage- 
cells (12B animal and 32 vegetative). (Haeckel.) 

Segmentation results in the formation of a multicellular 
aggregation ■which, sooner or later, incloses a central cavity 
(segmentation cavity, hlastocele). Usually this cell aggrega- 
tion {Uasiula, Uastosphere) is reduced to a single layer of in- 
vesting cells. 

The Gastrula. — Ensuing on the changes just described are 
others, which result in the formation of the gastrula, a form of 
cell aggregation of great interest from its resemblance to the 
Hydra and similar forms, which constitute in themselves inde- 
pendent animals that never pass beyond that stage. The blas- 
tula becomes flattened at one pole, then depressed, the cells at 


PLATE I. GASTRULATION. (After Haeekel.) 

Pigs. 1 to 17 represent holoblastic eggs (with total cleavage) ; Figs. 18 to 30 show meroblastie eggs (with partial cleavage). The 
animal halves are colored gray, the vegetative halves red. The nutritive yolk is shaded vertically. All the figures show vertical merid- 
ian sections through the axis of the primitive intestine. In all, the letters indicate the same parts : c, the parent-cell {cytula) ; /, cleav- 
age-cells (segmentella) ; to, the mulberry-germ {morula) ; 6, the germ-vesicle (blastula) ; g, the cup-germ (gastrula) ; s, the cleavage-cavity ; 
d, the primitive intestinal cavity ; o, the primitive mouth ; «, the nutritive yolk ; i, the intestinal layer ; e, the skin-layer. 

Figs. 1-6. — Original or primordial egg-cleavage of the lowest vertebrate (amphioxus). Fig. 1, parent- cell (cytula); Fig. 3, cleavage- 
stage with 4 cleavage-cells; Fig. 8, mulberry-germ {morula); Fig. 4, germ- vesicle (blastula); Pig. 5, the same, in process of inversion 
(invaginaiio) ; Fig. 6, bell-gastrula {archigastrula). 

Pigs. 7-11. — Unequal egg-cleavage of an amphibian (frog). Pig. 7, parent-cell {cytula); Pig. 8, cleavage-stage with 4 cleavage-cells; 
Pig. 9, mulberry-germ {morula) ; Pig. 10, germ-vesicle {blastula) ; Fig. 11, hood-gastrula {amphigastrula). 

Figs. 13-17. — Unequal egg-cleavage of a mammal (man). Pig. 12, parent-cell {cytula) ; Pig. 13, cleavage-stage with 3 cleavage-cells 
(e, mother-cell of the exoderm ; i, mother-cell of the entoderm) ; Pig. 14, cleavage-stage with 4 cleavage-cells ; Fig. 15, beginning of the 
inversion of the germ-vesicle; Pig. 16, further advanced inversion; Pig. 17, h.ooA.-^a.stxn\& {amphigastrula). 

Figs. 18-24. — Discoidal egg-cleavage of an osseous fish {Motella ? Cottus f). The greater part of the nutritive yolk (n) is omitted. 
(Cf. Figs. 43, 43, pp. 317, 319, Haeckel's "Pvolution of Man.") Pig. 18, parent-cell {cytula); Pig. 19, cleavage-stage with 3 cells; Pig. 30, 
cleavage-stage with 83 cells ; Pig. 31, mulberry-germ {morula) ; Fig. 33, germ-vesicle (blastula) ; Pig. 23, the same, in process of inver- 
sion; Pig. 24, disc-gastrula {disoogastrula). 

Pigs. 25-30. — Superficial egg-cleavage of a crab {peneus). Fig. 35, parent-cell {cytula) ; Fig. 36, cleavage-stage with 4 cells ; Pig. 37, 
cleavage-stage with 32 cells; Pig. 28, mulberry-germ (morula), and at the same time the germ-vesicle (blastula); Pig. 29, bladder-gas- 
trula {perigastrula) ; Pig. 80, nauplius-germ ; the pharynx-cavity has formed m front of the primitive mouth {d), owing to an inversion 
from without. 


this region becoming more columnar, {histological differentia- 
tion). This depression (invagination) deepens until a cavity is 

Fio. 70.— Blastula and gastrula of amphioxus (Qaus, after Hatschek). A, blastula with flat,- 
tened lower pole of larger cells ; B, commencing invagination ; C, gastrulation completed': 
the blastopore is still widely open, and one of the two hinder-pole mesoderm cells is seen 
at its ventral lip. The cilia of the epiblast cells are not represented. 

formed (as when a hollow rubber ball is thrust in at one part 
till it meets the opposite wall), in consequence of which a two- 
layered embryo results, in which we recognize the primitive 
mouth (blastopore) and digestive cavity (archenteron), the 
outer layer (ectoderm) being usually separated from the inner 
(endoderm) by the almost obliterated segmentation cavity. 
Such a form may be provided with cilia, be very actively loco- 
motive, and bear, consequently, the greatest resemblance to the 
permanent forms of some aquatic animals. 

The changes by which the segmented oosperm becomes a 
gastrula are not always so direct and simple as in the above- 
described case, but the behavior of the cells of the blastosphere 
may be hampered by a burden of relatively foreign matter, in 
the form of food-yelk, in certain instances ; so much so is this 
the case that distinct modes of gastrula formation may be rec- 
ognized as dependent on the quantity and arrangemept of food- 
yelk. These we shall pass by as being somewhat too compli- 
cated for our purpose, and we return to the egg of the bird. 

The Hen's Egg. — By far the larger part of the hen's egg is 
made up of yelk ; but just beneath the vitelline membrane a 
small, circular, whitish body, about four millimetres in diame- 
ter, which .always floats uppermost in every portion of the egg, 
may be seen. This disk (blastoderm, cicatricula) in the fertilized 
egg presents an outer white rim (area opaca), within which is 
a transparent zone (area peUucida), and most centrally a some- 
what elongated structure, which marks off the future being 
itself (embryo). All of these parts together constitute that por- 
tion (blastoderm) of the fowl's egg which is alone directly con- 
cerned in reproduction, all the rest serving for nutrition and 



protection. The appearance of relative opacity in some of the 
parts marked off as ahove is to be explained by thickening in 

the cell-layers of which they are 

The Origin of the Fowl's Egg.— 
The ovary of a young but mature 
hen consists of a mass of connect- 
ive tissue {stroma), abundantly, 
supplied with blood-vessels, from 
which hang the capsules which 
contain the ova in all stages of 
development, so that the whole 
suggests, but for the color, a bunch 
of grapes in an early stage. The 
ovum at first, in this case as in all 
others, a single cell, becomes com- 
plex by addition of other cells {dis- 
cus proligerus, etc.), which go to 
make up the yelk. All the other 
parts of the hen's egg are additions 
made to it, as explained before, in 
its passage down the oviduct. The 
original ovum remains as the blas- 
toderm, the segmentation of which 
may now be described briefly, its 
character being obvious from an 
examination of Fig. 72, which rep- 
resents a surface view of the seg- 
menting fertilized ovum {oosperm). 
A segmentation cavity appears 
early, and is bounded above by a 
single layer of epiblast cells and 
below by a single layer of primi- 
tive hypoblast cells, which latter 
is soon composed of several layers, 
while the segmentation cavity dis- 

The blastoderm of an unincu- 
bated but fertilized egg consists 
of a layer of epiblastic cells, and 
beneath this a mass of rounded 
cells, arranged irregularly and ly- 
ing loosely in the yelk, constitut- 

FiG. 71— Female generative organs of 
the fowl (after Dalton). A, ovary; 
5, Graafian follicle, from which the 
egg has just been discharged ; C, 
yelk, entering upper extremity of 
oviduct ; D, E, second portion of 
oviduct, in which the chalaziferous 
membrane, chalazae, and albumen 
are formed ; F^ third portion, in 
which the fibrous shSll membranes 
are produced ; (?, fourth portion laid 
open, showing the egg completely 
formed with its calcareous shell ; H, 
canal through which the egg is ex- 



t 111 11 1' ' 
Fig. 72, — Various stages in tlie segmentation of a tow 1 s egg (Kulliker). 

ing the primitive hypoblast. After incubation for a couple of 
hours, these cells become differentiated into a lower layer of 
flattened cells {hypoblast), with mesoblastic cells scattered be- 
tvsreen the epiblast and hypoblast. It is noteworthy that, in the 
bird, segmentation will proceed up to a certain stage indepen- 
dently of the advent of the male cell, apparently indicating a 
tendency to parthenogenesis. 

Fig. 73. — Portion of section tlirough an uniucubat«d fowl's oosperm (after Klein), a, epiblast 
composed of a single layer of columnar cells ; h, irregularly disposed lower layer cells of 
the primitive hypoblast : c. larger formative cells resting on white yelk ; /, archenteron. 
The segmentation cavity lies between a and 6, and is nearly obliterated. 



The fowl's ovum then belongs to the class, a portion of which 
alone segments and develops into the embryo {merohlastic), in 
contradistinction to what happens in the mammalian ovum, the 
whole of which undergoes division {holoblastic) ; a distinction 
which is, however, superficial rather than fundamental, for in 
reality in the fowl's egg the whole of the original ovum does 



FiQ. 74.— Sections of ovum of a rabbit, illustrating formation of the plastodermic vesicle (after 
E. Van Beneden). A, B, C, Di-are ova in successive stages of development, zp, zona pellu- 
cida : ect, eotomeres, or outer cells : ent, entomeres, or inner cells. 

segment. This holoblastic character of the mammalian ovum 
and its resemblance to the segmentation of those invertebrate 
forms previously described may become apparent from an ex- 
amination of the accompanying figures. 

"We shall return to the development of the mammalian ovum 
later ; in the mean time we present the main features of devel- 
opment in the bird. 

Remembering that the development of the embryo proper 
takes place within the pellucid area only, we point out that the 
area opaca gradually extends over the entire OATim, inclosing 



the yelk, so that the original disk which lay like a watch-glass 
on the rest of the ovum, has grown into a sphere. That portion 
of this area nearest the pellucid zone {area vasculosa) develops 

Fig. 75. — Diagrammatic transverse sections through a hypothetical mammal oSsperm (Had- 
don). A. The yelk of the primitive mammalian oosperm is now lost. B. Later stage ; 
the non-embryonic epiblast has gi-owu over the embryonic area to form the covering cdls. 
ep, epiblast of embryo ; ep\ epiblast of yelk-sac ; Ay, primitive hypoblast ; y. s, yelk-sac, 
or blastodermic vesicle. 

blood-vessels that derive the food-supplies, which replenish the 
blood as it is exhausted, from the hypoblast of the area opaca. 

The first indications of future structural outlines in the 
embryo is the formation of the primitive streak, an opaque band 
in the long diameter of the pellu- 
cid area, opaque in consequence 
of cell accummulation in that re- 
gion. Very soon a groove {primi- 
tive groove) extends throughout 
this band, which gradually occu- 
pies a more central position. The 
relative thickness of the several 
parts and the arrangement of cells 
may be gathered from Fig. 76. 
These structures are only tempo- 
rary, and those that replace them 
will be described subsequently. 

We have thus far spoken of 
cells as being arranged into epi- 
blast, hypoblast, and mesoblast. 
The origin of the first two has 
been sufficiently indicated. The 
mesoblast forms, the intermediate 
germinal layer, and is derived 
from the primitive hypoblast, 
which differentiates into a stratum of flattened cells, situated 
below the others, and constituting the later hypoblast, and in- 

FiG. 76.— Surface view of pellucid area of 
blastoderm of eighteen hours (Foster and 
Balfour), if, medullary folds; mc, med- 
yllary groove ; pr, primitive groove. 



termediate less closely arranged cells, termed, from tteir posi- 
tion, mesoblast. 

It will be noticed that all future growth of the embryo be- 
gins axially, at least in the early stages of its development. 

As the subsequent growth and advance of the embryo de- 
pend on an abundant and suitable nutritive supply, we must 
now turn to those arrangements which are temporary and of 
subordinate importance, but still for the time essential to devel- 

Embryonic Membranes op Birds. 
It will be borne in mind throughout that the chief food-sup- 
ply for the embryo bird is derived from the yelk ; and, as would 




-rr-^-: \y 

Figs. 77-79.— A series of diagrams intended to facilitate the comprehension of the relations of 
the membranes to other parts (after Foster and Balfour). A, B, C, D, E, F are vertical 
sections in the long axis of the embryo at different periods, showing the stages of develop- 
ment of the amnion and of the yelk-sac. I, U, III, IV are transverse sections at about the 
same stages of development, i, ii, iii, posterior part of lon^tudinal section, to illustrate 
three stages in formation of the allantois. e, embryo ; i/, ydk ; pp, pleuroperitoneal cav- 
ity ; vt, vitelline membrane of amniotic fold ; oi, allantois ; a, amnion ; a', alimentary 

be expected, the older the embryo the smaller the yelk, or, as it 
is now called when limited by the embryonic membranes, the 
yelk-sac {umbilical vesicle of the mammalian embryo). The 
manner in which this takes place will appear upon an inspec- 
tion of the accompanying fig-ures. 

Very early in the history of the embryo two eminences, the 
head and the tail folds, arise, and, curving over toward each 





otter, meet after being joined by corresponding lateral folds. 
Fusion and absorption result at this meeting-point, in the 
inclosure of one cavity and the blending of two others. These 


Fig. 80. —Diagrammatic longitudinal section through the axis of an embryo chick (after Foster 
and Balfour). N. C, Neural canal ; CA, notochord ; Fg, foregut ; F. So, somatopleure ; 
F. Sp, splanchnopleure ; Sp, splanchnopleure, forming lower wall of foregut ; Ht, heart ; 
pp, pleuroperitoneal cavity ; Am, amniotic fold ; E, epiblast ; M, mesoblast ; H, hypoblast. 

folds constitute the amniotic membranes, the inner of which 
forms the true amnion, the outer the false amnion {serous m,em- 
hrane, subzonal membrane). Within the amnion proper is the 
amniotic cavity filled with fluid {liquor amnii), while the space 
between the true and false amniotic folds, which gradually in- 
creases in size as the yelk-sac diminishes, forms the pleuro- 
peritoneal cavity, body cavity, or ccelom. The amniotic cavity 
also extends, so that the embryo is surrounded by it or lies 
centrally within it. The enlargement of the coelom and exten- 
sion of the false amniotic folds lead finally to a similar meeting 
and fusion like that which occurred in the formation of the true 
amniotic cavity. The yelk-sac, gradually lessening, is at last 
withdrawn into the body of the embryo. 

Fig. 80 shows how the amniotic head fold arises, from a 
budding out of the epiblast and mesoblast at a point where the 
original cell layers of the embryo have separated into two folds, 
the somatopleure or body fold and the splanchnopleure or vis- 
ceral fold, owing to a division or cleavage of the mesoblast 
toward the long axis of the body. Remembering this, it is 
always easy to determine by a diagram the composition of any 
one of the membranes or folds of the embryo, for the compo- 
nents must be epiblast, mesoblast, or hypoblast ; thus, the 
splanchnopleure is made up of hypoblast internally and meso- 
blast externally — a principle of great significance, since, as will 
be learned later, all the tissues of the body may be classified 
simply, and at the same time scientifically, according to their 
embryological origin. 



Tlie allantois is a structure of much, physiological impor- 
tance. It arises at the same time as the amniotic folds are 
forming, by a budding or protrusion of the hind-gut into the 

Fig, 81. — Diagrammatic longitudinal section of a cMcIj: of tlie fourtli day (after Allen Thom- 
son), ep, epiblast ; hy, hypoblast : am, somatopleure ; mn, splanchnopleure ; a/, pf, folds 
of the amnion ; pp, pleuroperitoneal cavity ; arn^ cavity of the amnion ; at, allantois ; a, 
position of the future anus ; ft, heart ; i, intestine ; vi, vitelline duct ; ys, yelk ; s, f oregut ; 
m, position of the mouth ; me, mesentery. 

pleuro-peritoneal cavity, and hence consists of an outgrowth 
of mesoblast lined by hypoblast. 

The outer membrane of the allantois fuses with the subzo- 
nal (serous) membrane, and, with the latter extending beyond 
the yelk-sac, incloses the albumen of the egg in a space termed 

= — ==w<i!. m. 

FiQ. 8S. — A. Diagrammatic longitudinal section through the egg of a fowl. B. Detail of por- 
tion of same at a time when the allantois reached the spot marked x in A (after Duval). 
ah cavity of allantois ; aW), albumen ; ali, mesenteron ; al, hy^ hypoblastic epithelium of 
allantois ; al. m, mesoblast of allantois ; am, cavity of amnion ; 6, blood-vessel ; emb, 
embryo ; ep, epiblast of outer layer of amnion (serous membrane) ; ep. am, epiblastic 
epitheUum of inner layer of amnion (amnion proper) ; m. am, mesoblastic layer of latter ; 
sh, e^g-shell ; som, somatic mesoblast of outer layer of amnion ; v. m, vitelline membrane ; 
X, point where the mesoblastic tissue of the allantois fuses with Uiat of the serous mem- 



the placental sac by Duval, wlio has recently described this pro- 
cess. Villi, or tubular vascular outgrowths, spring from the 
lining of this sac and serve to convey the absorbed and prob- 
ably altered albumen to the embryo, in which process of vas- 
cular transport of nourishment the yelk-sac, that also abounds 
in blood-vessels as well as the allantois, takes part. The 
physiological import of the various structures above described 
will be considered more fully later. At this point a compari- 
son of the formation of the corresponding parts in mammals 
will be undertaken. 

The Fcbtal (Embryonic) Membranes of Mammals. 

The differences between the development of the egg mem- 
branes of mammals and birds are chiefly such as result from 

Fig. 83. 

Via. 84. 

Fig. 83. — Diagrammatic loiig;itudinal section of oSsperm of rabbit at an advanced stage of 
pregnancy (KoUiker, after Bischoff). o, amnion ; ai, allantois with its blood-vessels ; e, 
embryo ; ds, yelk-sac ; ed, ed\ ed'\ bypoblastic epithelium of the yelk-sac and its stalk 
(umbilical Vesicle and cord) ; /d, vascular mesoblastic membrane of the umbilical cord 
and vesicle ; p, placental villi formed by the allantois and subzonal membrane ; r, space 
fjled with fluid between the amnion, the allantois, and the yelk-sac ; si, sinus terminalis 
(marginal vitelline blood-vessel) ; it, urachus, or stalk of the allantois. 

Fio. 84. — Diagrammatic dorsal view of an embryo rabbit with its membranes at the stage of 
nine somites (Haddon, after Van Beneden and Julin). al, allantois, showing from bSiind 
the tail fold of the embryo ; am. anterior border of true amnion ; a. v, area vasculosa, the 
outer border of which Indicates the farthest extension of the mesoblast ; bi, blastoderm, 
here consisting only of epiblast and hypoblast ; o. m. v, omphalo-raesenteric or vitelline 
veins ; p. am, proamnion ; pi, non-vascular epiblastic villi of the future placenta ; s. t, si- 
nus terminalis. 

the absence in the former of an egg-shell and its membranes, 
and of yelk and albumen. The mammalian ovum is inclosed 
by a zona radiata {zona peUucida) surrounding another very 
delicate covering (Fig. 58). 

The growth of the blastodermic vesicle (yelk-sac) is rapid. 



and, being filled with fluid, tlie zona is thinned and soon disap- 

The germinal area alone is made np of three layers of cells 
(Fig. 104), the rest of the upper part of the oosperm being lined 
with epiblast and hypoblast, while the lower zone of the yelk- 
sac consist's of epiblast only. 

Simple, non-vascular villi, serving to attach the embryo 
to the uterine walls, usually project from the epiblast of the 
subzonal membrane. In the rabbit they do not occur every- 
where, but only in that region of the epiblast beneath which the 
mesoblast does not extend, with the exception of a patch which 
soon appears and demarkates the site of the future placenta. 

The extension of the mesoblast takes place in every direction 
from the embryo except directly around the head ; but the two 

FiQ. 85.— Diagrammatic median vertical longitudinal sections through embryo rabbit (Had- 
don, after Van Beneden and Julin). A. Section through embryo of Fig. 84. B. Section 
through embryo of eleven days, ai, allantois ; am, aminion ; a. ms, anterior median plate 
of mesoblast, formed by the junction of the anterior horns of the area opaca ; o. pi, area 
placentalis ; a. v, area vasculoaa ; c7i, chorion ; cos, coelom of embryo : cce', extra-embry- 
onic portion of the body-cavity ; ep, epiblast ; hy, hypoblast ; m, unsjmt mesoblast ; o. a, 
orifice of amnion ; pi, placenta ; pro. a, proamnion ; s. t, sinus terminalis ; v, epiblastic 
villi of blastodermic vesicle. 

expansions of the mesoblast which mark out this area extend 
for some distance in front of the head, and ultimately unite ; 
so that immediately in front of the head there is a circular 
region in which the blastoderm consists of epiblast and hypo- 


blast only, forming a cavity into whicli the anterior part of tlie 
embryo early projects (pro-amnion). 

The true amnion arises only from the posterior end of the 
embrj'^o, and, extending over in a forward direction, meets the 
raised projection of the pro-amnion with which it fuses. 

The amniotic cavity becomes one with that space (extra-em- 
bryonic pleuro-peritoneal cavity) arising from the cleavage of 

Fig. 86. — Foetal envelopes of a rabbit embryo (Minot, after Van Beneden and Julin). Later 
stage than Fig. io B. The amnion has become fused with the blastoderm in front of the 
embryo, and its cavity is therefore continuous ■with the extra-embryonic portion of the 
body-cavity in front of the embryo. Al^ allantois ; am^ amnion ; ani\ portion of the 
amnion united with the walls of the allantois ; A.pU area placentalis ; Av^ area vasculosa ; 
Ch, chorion ; Coe, coelom or body-cavity ; Cos", extra-embryonic portion of the body- 
< cavity ; Ccel, anterior portion of the same, produced by the fusion of the cavity of the 
amnion with that of the anterior portion of the area opaca ; Ec^ epiblast ; jEit, alimentary 
canal of the embryo ; Ent, hypoblast ; PI, placenta ; pro. A, proamnion ; T, sinus ter- 
minalis : V, villi of blastodermic vesicle ; Y, cavity of blastodermic vesicle. 

the mesoblast, which now advances beyond the head of the em- 
bryo and the pro-amnion. The pro-amnion by gradual atrophy 
gives place to the true amnion. 

At about the same period as these events are transpiring the 
vascular yelk-sac has become smaller, and the allantois with its 
abundant supply of blood-vessels is becoming more prominent, 
and extending between the amnion and subzonal membrane. 

The formation of the chorion marks an important step in 
the development of mammals in which it plays an important 
functional part. It is the result of the fusion of the allantois, 
which is highly vascular, with the subzonal membrane, the villi 
of which now become themselves vascular and more complex 
in other respects. 

An interesting resemblance to birds has been observed (by 
Osborn) in the opossum. When the allantois is small the 



Fia. 87.— Embryo of dog, twenty-flve days old, opened on the ventral side. Chest and ven- 
tral walls have been removed, a, nose-pits ; 6, eyes ; c, under-jaw (first gill-arch) ; d, 
second gill-arch ; e, /, o, 7i. heart (e, right, /, leift auricle ; g, right, h, left ventricle) ; i, 
aorta (origin of) ; J;*:, liver (in the middle between the two lobes is the out yelk-vein) ; 
Z, stomach ; m, intestine ; n., yelk-sac ; o, primitive kidneys : p, allantois ; g, rore-limbs ; 
/i, hind-limbs. The crooked embryo has been stretched straight. ' (Haeckel, after 

. -sxJMUp 

^~^ir~^K ^ ^'""- 

Fio. 88.— Diagram of an embryo showing the relations of the vascular allantois to the villi of 
the chorion (Cadiat). e, embryo lying in the cavity of the amnion ; «s, yelk-sac ; aZ, al- 
lantois ; A. V, allantoic vessels dipping into the vilU of the chorion ; en, cnorion. 



blastodermic vesicle (yelk-sac) has vascular villi, which in all 
probability not only serve the purpose of attaching the embryo 
to the uterine wall but derive nourishment, not as in birds, from 
the albumen of the ovum, but directly in some way from the 
uterine wall of the mother. It will be remembered that the 

opossum ranks low in the 
mammalian scale, so tha^ this 
resemblance is the more signifi- 
cant from an evolutionary point 
of view. 

The term chorion is now re- 
stricted to those regions of the 
subzonal membrane to which 
either the yelk-sac or the allan- 
tois is attached. The former 
zone has been distinguished as 
the false chorion and the latter 
the true chorion. In the 

Fig. 89.— Diagram of the foetal membranes of aS 

the Virginian opossum (Haddon, after Os- 

born). Two villi are shown greatly en- 

larged. The processes of the cells, which 
have been exaggerated, doubtless corre- 
spond to the pseudopodia described by 
Caldwell. aZ, allantois ; am^ amnion ; s. i, 
sinus terminalis ; 5. z, subzonal mem- 
brane ; ■!), viUi on the subzonal membrane 
in the region of the yelk-sac ; ys^ yelk 
sac. The vascular splanchnopleure (hy- 
poblast and mesoblast) is indicated by 
the black line. 

rabbit the false chorion is very 
large (Fig. 83), and the (placen- 
tal) chorion very small in com- 
parison, but the reverse is the 
case in most mammals. It will 
be noted that in both birds and 
mammals the allantois is a nu- 
tritive organ. Usually the more prominent and persistent the 
yelk-sac, the less so the allantois, and ■ vice versa j they are 
plainly supplementary organs. 

The Placenta. — This structure, which varies greatly in com- 
plexity, may be regarded as the result of the union of structures 
existing for a longer or shorter period, free and largely inde- 
pendent of each other. With evolution there is differentiation 
and complication, so that the placenta usually marks the site 
where structures have met and fused, differentiating a new 
organ; while corresponding atrophy, obliteration, and fusion 
take place in other regions. 

All placentas are highly vascular, all are villous, all dis- 
charge similar functions in providing the embryo with nourish- 
ment and eliminating the waste of its cell-life (metabolism). 
In structural details they are so different that classifications of 
mammals have been founded upon their resemblances and dif- 
ferences. These will now be briefly described. 

In marsupials the yelk-sac is both large and vascular ; the 


allantois small but vascular ; the former is said (Owen) to be 
attached to the subzonal membrane, the latter not ; but no villi, 
and consequently no true chorion, is developed. All mammals 
other than the monotremes and marsupials have a true allan- 
toic placenta. ' 

The Discoidal Placenta. — This form of placenta is that existing 
in the rodentia, insectivora, and cheiroptera. The condition 
found in the rabbit is that which has been most studied. The 
relation of parts is shown in Fig. 83. 

The uterus of the rodent is two-horned ; so we find in gen- 
eral several embryos in each horn in the pregnant rabbit. They 
are functionally independent, each having its own set of mem- 
branes. It will be observed from the figure that the true vil- 
lous chorion is confined to a comparatively small region ; there 
is, however, in addition a false chorion without villi, but highly 
vascular. This blending of forms of placentation which exist 
separately in different groups of animals is significant. In the 
rabbit, at a later stage, there is considerable intermingling of 
f cetal and maternal parts. 

The Metadiscoidal Placenta. — This type, which, in general 
naked-eye appearances, greatly resembles the former, is found 
in man and the apes. The condition of things in man is by no 
means as well understood as in the lower mammals, especially 
in the early stages ; so that, while the following account is that 
usually given in works on embryology, the student may as well 
understand that our knowledge of human embryology in the 
very earliest stages is incomplete and partly conjectural. The 
reason of this is obvious : specimens for examination depending 
on accidents giving rise to abortion or sudden death, often not 
reaching the laboratory in a condition permitting of trust- 
worthy inferences. 

It is definitely known that the ovum, which is usually fer- 
tilized in the oviduct (Fallopian tube), on entering the uterus 
\ becomes adherent to its wall and encapsuled. The mucous 
membrane of the uterus is known to undergo changes, its com- 
ponent parts increasing by cell multiplication, becoming in- 
tensely vascular and functionally more active. The general 
mucous surface shares in this, and is termed the decidua vera ; 
but the locality where the ovum lodges is the seat of the great- 
est manifestation of exalted activity, and is termed the decidiia 
serotina; while the part believed to have invested the ovum by 
fused growths from the junction of the decidua vera and sero- 
tina is the decidua reflexa. 




The decidua serotina and reflexa thus become the outermost 
of all the coverings of the ovum. These and somie other devel- 
opments are figured below. It is to be remembered, however, 
that they are highly diagrammatic, and represent a mixture 

Fia. 90.— Series ot diagrams representing the relations of the decidua to the ovum, at different 
periods, in the human subject. The decidua are dark, the ovum shaded transversely. In 
4 and 5 the chorionic vascular processes are figured (after Dalton). 1. Ovum resting on 
the decidua serotina; 2. Decidua reSexa growing round the ovum; 3. Completion of the 
decidua around the ovum ; 4. Villi, growing out all around the chorion ; 5. The villi, spe- 
cially developed at the site of the future placenta, having atrophied elsewhere. 

of inferences based, some of them, on actual observation and 
others on analogy, etc. 

The figures will convey some information, though appear- 
ances in all such cases must be interpreted cautiously for the 
reasons already mentioned. 

During the first fourteen days villi appear over the whole 
surface of the ovum ; about this fact there is no doubt. At 
the end of the first month of foetal life, a complete chorion 
has been formed, owing, it would seem, to the growth of the 
allantois (its mesoblast only) beneath the whole surface of the 
subzonal membrane. From the chorionic surface vascular pro- 
cesses clothed with epithelium project like the plush of velvet. 



The allantois is compressed and devoid" of a cavity, but abun- 
dantly supplied with blood-vessels by the allantoic arteries and 

Kg. 91.— Vascular system of the human tcBtus, represented diagrammatically (Huxley). 
H, heart ; TA, aortic trunk ; c, common carotid artery ; c', external carotid artery ; c", 
internal carotid artery ; s, subclavian artery ; v, vertebral artery ; 1, 2, 3, 4, 5, aortic 

arnhps '. A', rlnren.1 nrtrf.». ■ a nmnhn.ln-mAS«nt«rin artrf^rv : fj.ii. vitfilfine duct '. n' . omphalo- 

arteries ; 

arches ; A', dorsal aorta ; o, omphalo-mesenteric artery : dv. vitelline duct ; o', 
mesenteric vein ; v\ umbilical vesicle ; up, portal vein ; i, liver ; iz, w, umbilical 
u"_, u", their endings in the placenta ; «', umbilical vein ; Dv, ductus venosus ; vh, hepatic 



cv, inferior' vena cava ; vz7, iliac veins 
DC, duct of Cuvier ; P, lung. 

az, vena azygos ; vc', posterior cardinal 

veins, which of course terminate in capillaries in the villi. 
Compare the whole series of figures. 

Fio. 92.— Human ova during early stages of development, A and B, front and side view of an 
ovum supposed to be about thirteen days old ; e, embryonic area (Quain, after Reichert) ; 
C, ovum or four to live weeks, showing the general structure of the ovum before formation 
of the placenta. Fart of th& wall of the ovum is removed to show the embryo in position 
(after Allen Thomson). 

At this stage the condition of the chorion suggests the type 
of the diffuse placenta which is normal for certain groups of 
animals, as will presently be learned. 

The subsequent changes are much better understood, for 



parts are in general no "longer microscopic but of considerable 
size, and tbeir real structure less readily obscured or obliterated. 
The amniotic cavity continues to enlarge by growth of the 
walls of the amnion and is kept filled with a fluid ; the yelk-sac 
is now very small ; the decidua reflexa becomes almost non- 
vascular, and fuses finally with the decidua vera and the cho- 
rion, which except at one part has ceased to be villous and vas- 
cular ; so that becoming thinner and thinner with the advance 
of pregnancy, the single membrane, arising practically from 
this fusion of several, is of a low type of structure, the result of 

Fig. 93.— Human embryo, twelve weeks old, with its coverings ; natural size. The navel-cord 
passes from the navel to the placenta, b, amnion ; c, chorion ; d, placenta ; (2', remains 
of tufts on the smooth chorion ; /, decidua reflexa (inner) ; g, decidiut vera (outer). (Haec- 
kel after Bemhard Schiiltze.) • 

gradual degeneration, as the rdle they once played was taken 
up by other parts. 

But of paramount importance is the formation of the pla- 
centa. The chorion ceases to be vascular except at the spot at 
wjiich the villi not only remain, but become more vascular and 
branch into arborescent forms of considerable complexity. It 
is discoidal in form, made up of a foetal part just described and 



a maternal part, the decidua serotina, tlie two becoming blended 
so that the removal of one involves that of more or less of the 
others. The connection of parts is far closer than that described 
for the rabbit ; and, even with the preparation that Nature makes 
for the final separation of the placenta from both foetus and 

Fig. 94.— Diagram illustrating the decidua, pilacenta, etc. (after Li^geois). e, embryo ; 
U intestine ; p, pedicle of the umbiUcal vesicle : u. v, umbilical vesicle ; a, amnion ; c/i, 
chorion; v. ty vascular tufts of the chorion, constituting the fcetal portion of the placenta; 
m. p, maternal portion of the placenta ; d. v, decidua vera ; d. r. decidua renexa ; aj, 

mother, this event does not take place without some rupture of 
vessels and consequent haemorrhage. 

It is difficult to conceive of the great vascularity of the 
human placenta without an actual examination of this structure 
itself, which can be done after being cast off to great advan- 
tage when floating in water ; by which simple method also the 
thinness and other characteristics of the membranes can be 
well made out. 

The great vessels conveying the foetal blood to and from the 
placenta are reduced to three, two arteries and one vein. The 
villi of the placenta (chorion) are usually said to hang freely 


in the blood of the large irregular sinuses of the decidua sero- 
tina; but this is so unlike what prevails in other groups of 
animals that we can not refrain from believing that the state- 
ment is not wholly true. 

The Zonary Placenta. — In this type the placenta is formed 
along a broad equatorial belt, leaving the poles free. This form 
of placentation is exemplified in the carnivora, hyrax, the ele- 
phant, etc. 

In the dog, for example, the yelk-sac is large, vascular, does 
not fuse with the chorion, and persists throughout. A rudiment- 
ary discoid placenta is first formed, as in the rabbit ; this grad- 
ually spreads over the whole central area, till only the extremes 
(poles) of the ovum remain free ; villi appear, fitting into pits 
in the uterine surface, the maternal and foetal parts of the pla- 
centa becoming highly vascular and closely approximated. 
The chorionic zone remains wider than the placental. As in 
man there is at birth a separation of the maternal as well as 
festal part of the placenta — i. e., the latter is deciduate ; there is 
also the beginning of a decidua reflexa. 

The Diffuse Placenta. — As found in the horse, pig, lemur, etc., 
the allantois completely incloses the embryo, and it becomes 
villous in all parts, except a small area at each pole. 

The Polycotyledonary Placenta.— This form is that met with in 
ruminants, in which case the allantois completely covers the 
surface of the subzonal membrane, the placental villi being 
gathered into patches (cotyledons), which are equivalent to so 
many independent placentas. The component villi fit into cor- 
responding pits in the uterine wall, which is specially thickened 
at these points. When examined in a fresh condition, under 
water, they constitute very beautiful objects. 

Comparing the formation, complete development, and atro- 
phy (in some cases) of the various foetal appendages in mam- 
mals, one can not but perceive a common plan of structure, 
with variations in the preponderance of one part over another 
here and there throughout. In birds these structures are sim- 
pler, chiefiy because less blended and because of the presence 
of much food-yelk, albumen, egg-shell, etc., on the one hand, 
and the absence of a uterine wall, with which in t]lie mammal 
the membranes are brought into close relationship, on the other ; 
but, as will be shown later, whatever the variations, they are 
adaptations to meet common needs and subserve common ends. 



Microscopic Structure of the Placenta. 

This varies somewliat for different forms, though, in that 
there is a supporting matrix, minute (capillary) blood-vessels, 
and epithelial coverings to the foetal and maternal surfaces, the 
several forms agree. 


Figs. 96 to 101.— Diagrammatic representation of the minute structure of the placenta (Foster 
and Balfour, after Turner). F, fcetal ; M, maternal placenta ; e, epithelium of chorion ; 
e', epithelium of maternal placenta ; d, foetal blood-vessels ; tJ', maternal blood-vessels ; 
V, villus. 

Fig. 95.— Placenta in most generalized form. 

Fig. 96.— structure of placenta of a pig. 

Fio. 97.— Of a cow. 

Fig. 98.— Of a fox. 

FiQ. 99.— Of a cat. 

The pig possesses the simplest form of placenta yet known. 
The villi fit into depressions or crypts in the maternal uterine 
mucous membrane. The villi, consisting of a core of connective 



tissue, in which, capillaries abound, are covered with a flat epi- 
thelium; the maternal crypts correspond, being composed of 
a similar matrix, lined with epithelium and permeated by- 
capillary vessels, which constitute a plexus or mesh-work. It 
thus results that two layers of epithelium intervene between 
the maternal and foetal capillaries. 

The arrangement is substantially the same in the diffuse and 
the cotyledonary placenta. 

In the deciduate placenta, naturally, there is greater compli- 

In certain forms, as in the fox and cat, the maternal tis- 
sue shows a system of trabeculse assuming a meshed form, 
in which run dilated capillaries. These, which are covered 
with a somewhat columnar epithelium, are everywhere in 
contact with the foetal villi, which are themselves covered with 
a flat epithelium. 

6. p; 

Fig. 100. 

Fig. 101. 

Fig 100.— Placenta ot a sloth. FlaJ maternal epithelial cells shown in position on right side ; 
on left they are removed and dilated ; maternal vessel with its blood-corpuscles exposed. 

Fig. 101. — Structure of human placeuta : ds, decidua serotina ; *, trabeculse of serotina passing 
tb foetal villi ; ca, curling artery ; up, utero-placental vein ; x, prolongation ,of maternal 
tissue on exterior of villus, outside cellular layer e', which may represent either endothe- 
lium of maternal blood-vessels or delicate connective tissue of the serotina or both ; e' ma- 
ternal cells of the serotina. 

In the case of the sloth, with a more discoidal placenta, the 
dilatation of capillaries and the modification of epithelium 
are greater. 

In the placenta of the apes and of the human subject the 
most marked departure from simplicity is found. The maternal 


vessels are said to constitute large intercommunicating sinuses ; 
the villi may liang freely suspended in these sinuses, or be 
anchored to their walls by strands of tissue. There is believed 
to be only one layer of epithelial cells between the vessels of 
mother and foetus in the later stages of pregnancy. This, 
while closely investing the foetal vessels (capillaries), really 
belongs to the maternal structures. The significance of this 
general arrangement will be explained in the chapter on the 
physiological aspects of the subject. 

It remains to inquire into the relation of these forms to one 
another from a phylogenetic (derivative) point of view, or to 
trace the evolution of the placenta. 

Evolution. — Passing by the lowest mammals, in which the 
placental relations are as yet imperfectly understood, it seems 
clear that the simplest condition is found in the rodentia. 
Thus, in the rabbit, as has been described, both yelk-sac and 
allantois take a nutritive part ; but the latter remains small. 
In forms above the rodents, the allantois assumes more and 
more importance, becomes larger, and sooner or later predomi- 
nates over the yelk-sac. 

The discoidal, zonary, cotyledonary, etc., are plainly evolu- 
tions from the diffuse, for both differentiation of structure and 
integration of parts are evident. The human placenta seems 
to have arisen from the diffuse form ; and it will be remem- 
bered that it is at one period represented by the chorion with 
its villi distributed universally. 

The resemblance in the embryonic membranes at any early 
stage in man and other mammals to those of birds certainly 
suggests an evolution of some kind, though exactly along what 
lines that has taken place it is difficult to determine with exact- 
ness ; however, as before remarked, nearly all the complications 
of the higher forms arise by concentration and fusion, on the one 
hand, and atrophy and disappearance of parts once functionally 
active, on the other. 

Summary, — The ovum is a typical cell ; unspecialized in most 
directions, but so specialized as to evolve from itself compli- 
cated structures of higher character. The segmentation of the 
ovum is usually preceded by fertilization, or the union of the 
nuclei of male and female cells, which is again preceded by the 
extrusion of polar globules. In the early changes of the ovum, 
including segmentation, periods of rest and activity alternate. 
The method of segmentation has relation to the quantity and 
arrangement of the food-yelk. Ova are divisible generally 


into completely segmenting (holoblastic), and those that under- 
go segmentation of only a part of their substance (meroblastic) ; 
but the processes are fundamentally the same. 

Provision is made for the nutrition, etc., of the ovum, when 
fertilized (oosperm) by the formation of yelk-sac and allan- 
tois; as development proceeds, one becomes more prominent 
than the other. The allantois may fuse with adjacent mem- 
branes and form at one part a condensed and hypertrophied 
chorion (placenta), with corresponding atrophy elsewhere. The 
arrangemeut of the placenta varies in different groups of ani- 
mals so constantly as to furnish a basis for classification. What- 
ever the variations in the structure of the placenta, it is always 
highly vascular ; its parts consist of villi fitting into crypts in 
the maternal uterine membrane — both the villi and the crypts 
being provided with capillaries supported by a connective-tissue 
matrix covered externally by epithelium. The placenta in its 
different forms would -appear to have been evolved from the 
diffuse type. 

The peculiarities of the embryonic membranes in birds are 
owing to the presence of a large foOd-yelk, egg-shell, and egg- 
membranes ; but throughout, vertebrates follow in a common 
line of development,- the differences which separate them into 
smaller and smaller groups appearing later and later. The 
same may be said of the animal kingdom as a whole. This 
seems to point clearly to a common origin with gradual diver- 
gence of type. 


We now turn to the development of the body of the animal 
for which the structures we have been describing exist. It is 
important, however, to remember that the development of parts, 
though treated separately for the sake of convenience, really 
goes on together to a certain extent ; that new structures do not 
appear suddenly but gradually ; and that the same law applies 
to the disappearance of organs which are being superseded by 
others. To represent this completely would require lengthy de- 
scriptions and an unlimited number of cuts ; but with the above 
caution it is hoped the student may be able to avoid erroneous 
conceptions, and form in his own mind that series of pictures 
which can not be well furnished in at least the space we have 
to devote to the subject. But, better than any abstract state- 



ments or pictorial representations, would be tlie examination of 
a setting of eggs day by day during their development under a 

Fig. 102.— Various stages in the development of the frog from the egg (after Howes). 1. The 
segmenting ovum, showing first cleavage furrow. 2. Section of the above at right angles 
to the furrow. 3. Same, on appearance of second furrow, viewed slightly from above. 
4. The latter seen from beneath. 5. The same, on appearance of first horizontal furrow. 
6. The same, seen from above. 7. Longitudinal section of 6. 8 and 9. Two phases in 
segmentation, on appearance of fourth and fifth furrows. 10. Longitudinal vertical section 
at a slightly later stage than the above. 11. Later stage. Upper pigmented pole dividing 
more rapidly than lower. 12. Later phase of 11. 13. Longitudinal vertical section of 12. 
14. Segmenting ovum at blastopore stage. 15. Longitudmal vertical section of same. 
13 and 15 x 10 (all others x 5). Iti. Longitudinal vertical section of embryo at a stage 
later than 14 (1 x 10). nc, nucleus ; c. c, cleavage cavity ; ep, epiblast ; 1. 1, yelk-bearing 
lower-layer cells ; bl, blastopore ; al, archenteron (mid-gut) ; hb, hjrpoblast ; wis, undiffer- 
entiated mesoblast ; ch^ notochord ; n, a, neural (cerebro-spinal) axis. 

hen. This is a very simple matter, and, while the making and 
mounting of sections from hardened specimens is valuable, it 
may require m'&re time than 'the student can spare ; but it is 
neither so valuable nor so easily accomplished as what we have 
indicated ; for, while the lack of sections made by the student 


may be made up in part by the exhibition to him of a set of 
specimens permanently mounted or even by plates, nothing can, 
in our opinion, take the place of the lamination of eggs as we 
have suggested. It prepares for the study of the development 
of the mammal, and exhibits the membranes in a simplicity, 
freshness, and beauty which impart a knowledge that only 
such direct contact with nature can supply. To proceed with 
great simplicity and very little apparatus, one requires but a 
forceps, a glass dish or two, a couple of watch-glasses, or a 
broad section-lifter (even a case-knife will answer), some water, 
containing just enough salt to be tasted, rendered lukewarm 

Holding the egg longitudinally, crack it across the center 
transversely, gently and carefully pick away the shell and its 
membranes, when the blastoderm may be seen floating upward, 
as it always does. It shoiild be well examined in position, 
using a hand lens, though this is not essential to getting a fair 
knowledge ; in fact, if the exainination goes no further than 
the naked-eye appearances of a dozen eggs, selecting one every 
twenty-four hours during incubation, when opened and the 
shell and membranes well cleared away, such a knowledge will 
be supplied as can be obtained from no books or lectures how- 
ever good. It will be, of course, understood that the student 
approaches these examinations with some ideas gained from 
plates and previous reading. The latter will furnish a sort of 
biological pabulum on which he may feed till he can furnish 
for himself a more natural and therefore more healthful one. 
While these remarks apply with a certain degree of force to all 
the departments of physiology, they are of special importance 
to aid the constructive faculty in building up correct notions 
of the successive rapid transformations that occur in the de- 
velopment of a bird or mammal. 

Fig. 103 shows the embryo of the bird at a very early 
period, when already, however, some of the main outlines of 
structure are marked out. Development in the fowl is so rapid 
that a few days suffice to outline all the principal organs of 
the body. In the mammal the process is slower, but in the 
main takes place in the same fashion. 

As the result of long and patient observation, it is now set- 
tled that all the parts of the most complicated organism arise 
from the three-layered blastoderm previously figured ; every 
part may be traced back as arising in one or other of these lay- 
ers of cells — the epiblast, mesoblast, or hypoblast. It frequently 






■^ irr. 


-happens that an organ is made tip of cells derived from more 
than one layer. Structures may, accordingly, he classified as 
epihlastic, mesoblastic, or hypoblas- 
tic; for, when two strata of cells 
unite in the formation of any part, 
one is always of subordinate impor- 
tance to the other : thus the digestive 
organs are made up of mesoblast as 
well as hypoblast, but the latter 
■constitutes the essential secreting 
cell mechanism. As already indi- 
cated, the embryonic membranes 
are also derived from the same 

The epihlast gives rise to the skin 
and its appendages (hair, nails, feath- 
ers, etc.), the whole of the nervous 
system, and the chief parts of the or- 
gans of special sense. 

The mesoblast originates the skel- 
eton, all forms of connective tissue, 
including the framework of glands, 
the muscles, and the epithelial (en- 
dothelial) structures covering serous 

The hypoblast furnishes the se- 
creting cells of the digestive tract 
and its appendages — as the liver and 
pancreas-f-the lining epithelium of 
the lungs, and the cells of the secret- 
ing mucous membranes of their 
framework of bronchial tubes. 

It is difficult to overrate the im- 
portance of these morphological gen- 
eralizations for the physiologist ; for, 
once the origin of an organ is known, 
its function and physiological rela- 
tions generally may be predicted with 
considerable certainty. We shall en- 
deavor to make this prominent in the future chapters of this 

Being prepared with these generalizations, we continue our 
study of the development of the bird's embryo. Before the end 





Fia, 103. — Embryo fowl 3 mm, long, 
of about twenty-four hours, seen 
from above. 1 x 39. (Haddon, 
after K611iker.) Mn^ union of 
the medullary folds in the region 
of the hind-brain ; Pr. pri^pitive 
streak ; Pz^ parietal zone ; i2/, 
posterior portion of widely-open 
neural groove ; Rf'^ anterior part 
of neural groove ; iSw, neural 
ridge ; Stz^ trunk- zone : vAf^ an- 
terior amniotic fold ; i;Z>, anterior 
umbilical siniK showing through 
the blastoderm. His divides the 
embryonic rudiment into a cen- 
tral trunk-zone, and a pair of 
lateral or parietal zones. 



of the first twenty-four hours such an appearance as that repre- 
sented in Fig. 104 is presented. 


Fie. 104.— Transverse section through the medullary groove and half the blastoderm of a 
chick of eighteen hours (Foster and Balfour). E, epiblast ; M, mesoblast ; if, hypoblast ; 
Ml/, medullary told ; mg. medullary groove ; eft, notoehord. 

The mounds of cells forming the medullary folds are seen 
coming in contact to form the medullary (neural) canal. 

Fig. 105.— Transverse section of embryo chick at end of first day (after KBUiker). M, meso- 
blast ; H, hypoblast ; m, medullary plate ; ^, epiblast ; mg, medullary groove ; m/, me- 
dullary fold ; eft, chorda dorsalis ; P, protovertebral plate ; dm, division of mesoblast. 

The notoehord, marking oilt the future hony axis of the 
body, may also be seen during the first day as a well-marked 
linear extension, just beneath the medullary groove. The cleav- 

Fis. 106.— Transverse section of chick at end of second day (Kolliker). E, epiblast ; H. hypo- 
blast ; e.m, external plate of mesoblast dividing (cleavage of mesoblast) ; m. /, medullary 
fold ; m. g, medullary groove ; ao, aorta ; p, pleuroperitoneaJl cavity ; P, protovertebral 

age of the mesoblast, resulting in the commencement of the 
formation of somatopleure (body-fold) and the splanchnopleure 
(visceral fold), is also an early and important event. These give 
rise between them to the pleuro-peritoneal cavity. The portions 
of mesoblast nearest the neural canal form masses (vertebral 
plates) distinct from the thinner outer ones (lateral plates). 




The vertebral plates, wlien distinctly marked ofE, as repre- 
sented in the figure, are termed the protovertehrce, {mesoblastic 
somites), and represent the future vertebrae and the voluntary- 
muscles of the trunk ; the former arising from the inner sub- 
divisions, and the latter from 
the outer {muscle-plates). It 
will be understood that the pro- 
tovertebrse are the results of 
transverse division of the col- 
umns of mesoblast that formed 
the vertebral plates. 

Before the permanent verte- 
brae are formed, a reunion of 
the original protovertebrae takes 
place as one cartilaginous pillar, 
followed by a new segmentation 
midway between the original 

It thus appears that a large 
, number of structures either ap- 
pear or are clearly outlined dur- 
ing the first day of incubation : 
the primitive streak, primitive 
groove, medullary plates and 
groove, the neural canal, the 
head-fold, the cleavage of the 
mesoblast, the protovertebrse, 
with traces of the amnion and 
area opaca. 

During the second day near- 
ly all the remaining important 
structures of the chick are 
marked out, while those that 

arose during the first day have progressed. Thus, the medullary 
folds close ; there is an increase in the number of protoverte- 
bras ; the formation of a tubular heart and the great blood-ves- 
sels ; the appearance of the "Wolffian duct ; the progress of the 
head region ; the appearance of the three cerebral vesicles at 
the anterior extremity of the neural canal ; the subdivision of 
the first cerebral vesicle into the optic vesicles and the begin- 
nings of the cerebrum ; the auditory pit arising in the third 
cerebral vesicle (hind-brain) ; cranial fiexure commences ; both 
head and tail folds become more distinct ; the heart is not only 

Fig. 107.— Embryo of chick, between thirty 
and thirty-six hours, viewed from above 
as an opaque object (Foster and Balfour). 
/. 6, forebrain ; m. b, midbrain ; ft. 6, 
hind-brain ; op. v, oijtic vesicle ; au. p, 
auditory ^it ; o. /, vitelline vein ; p. v, 
mesoblastic somite ; m./, line of func- 
tion of medullary folds above medullary 
canal ; s. r, sinus rhomboidalis ; t, tail- 
fold ; p. r, remains of primitive groove ; 
a. p, area peUucida. 



formed, but its curvature becomes more marked and rudim.ents 
of auricles arise ; while outside the embryo itself the circula- 
tion of the yelk-sac is established, the allantois originates, and 
the amnion makes rapid progress. 

It may be noticed that the cerebral vesicles, the optic vesi- 
cles, and the auditory pit are all derived from the epiblastic 
accumulations which occur in the anterior extremity of the 
embryo ; and their early appearance is prophetic of their physi- 
ological importance. 

The heart, too, so essential for the nutrition of the embryo, 
by distributing a constant blood-stream, is early formed, and 

Fig. 108. — Diagram rei)resentiug under surface of an embryo rabbit of nine days and tlu'ee 
hours old, illustrating development of the heart (after Allen Thomson). A, view of the 
entire embryo ; B, an enlarged outline of the heart of A ; C, later stage of the development 
of B ; 7i h, ununited heart ; a a, aortae ; vv, vitelline veins. 

becomes functionally active. It arises beneath the hind-end of 
the fore-gut, at the point of divergence of the folds of the 
splanchnopleure, and so lies within the pleuro-peritoneal cav- 
ity, and is derived from the mesoblast. At the beginning the 
heart consists of two solid columns ununited in front at first ; 
later, these fuse, in part, so that they have been compared with 
an inverted Y, in which the heart itself would correspond to the 
lower stem of the letter (a) and the great veins (vitelline) to its 
main limbs. The solid cords of mesoblast become hollow prior 
to their coalescence, when the two tubes become one. 



The entire blood-vascular system originates in tlie mesoblast 
of th.e area opaca especially ; at first appearing in isolated spots 
whicli come together as actual vessels are formed. The student 
who will pursue the plan of examining a series of incubating 
eggs will be struck with the early rise and ramd progress of the 

Fio. 109.— Chick on third day, seen from beneath as a transparent object, the head being 
turned to one side (Foster and Balfour), a', false amnion ; a, amnion ; CH, cerebral 
hemisphere ; FB, MB, HB, anterior, middle, and posterior cerebral vesicles ; OP, optic 
vesicle ; ot, auditory vesicle ; ofv, omphalo-mesenteric veins ; Ht, heart ; Ao, bulbus arte- 
riosus ; eft, notoohord ; Of a, omphalo-mesenteric arteries ; Pv, protovertebrae ; x, point of 
divergence of the splancnnopleural folds ; y, termination of the foregut, v. 

vascular system of the embryo, which takes, when complete, 
such a form as is represented diagramatically in Fig. 113. 

The blood and the blood-vessels arise simultaneously from 
the cells of the mesoblast by outgrowths of nuclear prolifera- 
tion, and in the case of vessels (Fig. 147) extension of processes, 
fusion, and excavation. 

The fore-gut is formed by the union of the folds of the 
splanchnopleure from before backward, and the hind-gut in a 
similar manner by fusion from behind forward. 



The excretory system is also f oresliado-wed at an early pe- 
riod by the "Wolffian duct (Fig. 114), a mass of mesoblast cells 
near which the clearage of the mesoblast takes place. 

Fio. 110.— Diagram of the heart and principal arteries of the chick (Quain). A represents an 
earlier, and B and C later stages. 1, 1, omphalo-mesenteric veins ; 2, auricle; 3, ventricle; 
4, aortic bulb ; 6, 5, primitive aortse ; 6, 6, omphalo-mesenteric arteries ; A, united acrtse. 

During the latter part of the sfecond day the vascular system, 
including the heart, makes great progress. The latter, in con- 

PiG. III.— Diagrammatic outlines of the early arterial system of the mammal vertebrate em- 
bryo (aft^r Allen Thomson). A. At a period corresponding to the thirty-sixth or thirty- 
eighth hour of incubation. B. Later stage, with two pairs of aortic arches. A, bulbus 
arteriosus of heart ; v, vitelUne arteries ; 1-5, the aortic arches. The dotted lines indicate 
the position of the future arches. 

sequence of excessive growth and the alteration of the relative 
position of other parts, becomes bent up on itself, so that it 



presents a curve to the right which represents the venous part 
and one to the left, answering to the arterial. The rudiments 
of the auricles also are to be seen. 

The arterial system is represented at this stage by the ex- 
panded portion of the heart known as the bulbus arteriosus, 
and two extensions from it, the aortae, 
which uniting above the alimentary 
canal, form a single posterior or dorsal 
aorta. From these great arterial ves- 
sels the lesser ones arise, and by sub- 
division constitute that great mesh- 
work represented diagrammatically in 
Figs. 112, 113, from which the course of 
the circulation may be gathered. The 
beating of the heart commences be- 
fore the corpuscles have become nu- 
merous, and while the tubular system, 
through which the blood is to be 
driven, is still very incomplete. 

The events of the third day are of 
the nature of the extension of parts 
already marked out rather than the 
formation of entirely new ones. The 
following are the principal changes: 
The bending of the head-end down- 
ward (cranial flexure) ; the turning of 
the embryo so that it lies on its left 
side; the completion of the vitelline 
circulation ; the increase in the curva- 
ture of the heart and its complexity 
of structure by divisions ; the appear- 
ance of additional aortic arches and 
of the cardinal veins ; the formation 
of four visceral clefts and five vis- 
ceral arches; a series of progressive 
changes in the organs of the special 
senses, such as the formation of the 
lens of the eye and a secondary optic 
vesicle; the closing in of the optic 
vesicle ; and the formation of the na- 
sal pits. In the region of the future brain, the vesicles of the 
cerebral hemispheres become distinct ; the hind-brain separates 
into cerebellum and medulla oblongata ; the nerves, both cra- 


113. — Dia^am of the embry- 
onic vascular system rWieder- 
sheim). a, atrium ; A. A, dor- 
sal aorta ; Ab^ branctiial ves- 
sels ; Acd^ caudal artery ; All^ 
allantoic (hypogastric) arter- 
ies ; Am, vitelline arteries ; B, 
bulbus arteriosus ; c, c,' exter- 
nal and internal carotids ; Z), 
ductus Cuvieri (precaval veins) ; 
E, external ihac arteries ; H. C, 
posterior cardinal vein ; Ic, 
common iliac arteries ; K. L, 
gill clefts ; B. A, right and left 
roots of the aorta ; S. S', 
branchial collecting^ trunks or 
veins ; Sb, subclavian artery ; 
Sb\ subclavian vein ; Si, sinus 
venosus ; F, ventricle ; VC, an- 
terior cardinal vein ; Fm, vitel- 
line veins. 



nial and spinal, bud out from the nervous centers. Tlie ali- 
mentary canal enlarges, a fore-gut and hind-gut being formed, 
the former being divided into oesophagus, stomach, and duode- 


Fig. 113.— Diagram of circulation of yelk-sac at end of third day (Foster and Balfour). 
Blastoderm seen from below. Arteries made black. H, heart ; AA, second, third, and 
fourth aortic arches ; .40, dorsal aorta ; L. Of. A, left vitelline artery ; i?. Of. A^ right 
vitelline artery ; S. T, sinus terminalis ; L. Of, left vitelline vein ; B. Of, right vitelhne 

vein ; S. F, sinus venosus ; Z>. C ductus Cuvieri ; S. Ca. V, superior cardmaT or jugular 
vein ; V, Ca, inferior cardinal vein. 


num ; the latter into the large intestine and the cloaca. The 
lungs arise from the alimentary canal in front of the stomach ; 
from similar diveirticula from the duodenum, the liver and 
pancreas originate. Changes in the protovertebrae and muscle- 
plates continue, while the Wolffian bodies are formed and the 
Wolffian duct modified. 

Up to the third day the embryo lies mouth downward, but 
now it comes to lie on its left side. See Fig. 109 with the ac- 
companying description, it being borne in mind that the view is 
from below, so that the right in the cut is the left in the em- 



Fig. 116. 

Fig. 115. 

Fig. 114.— Transverse section through lumbar region of an embryo at end of fourth day (Fos- 
ter and Balfour), nc, neural canal ; pr^ posterior root of spinal nerve with ganglion ; 
ar, anterior root ; A. G. C anterior gray column of spinal cord : A. W. C, anterior white 
column in course of formation ; m. jp, muscle-plate ; ch. notochord ; W. -B, Wolffian ridge ; 
AO, dorsal aorta ; v. c. o, posterior cardinal vein ; W. d. Wolffian duct ; W. b, Wolflnan 
body, consisting of tubules and Malpighian corpuscles ; g. e, germinal epithelium ; d, ali- 
mentary canal ; M, commencing mesentery ; SO, somatopleure ; SP^ splanchnopleure ; 
V, blood-vessels ; »p, pleuroperitoneal cavity. 

Fig. 115.— Diagram of portion of digestive tract of chick on fourth day (after G-Stte). The 
black line represents hypoblast ; the shaded portion, mesoblast ; Ig, lung diverticulum, 
expanding at bases into primary lung vesicle ; st, stomach ; I, liver ; p, pancreas. 

Fig. 116. — Head of chick of third day, viewed sidewise as a transparent object (HuxleyJ. Ja, 
cerebral hemispheres ; 76, vesicle of third ventricle ; II, mid-brain ; III, hind-brain ; a, 
optic vesicle ; g, nasal pit ; 6, otic v^icle ; d, infundibulum ; e, pineal body ; /i, notochord; 
v , fifth nerve ; VII, seventh nerve ; VIII, united glossopharyngeal and pneumogastric 
nerves. 1, 3, 3, 4, 5, the five visceral folds. 



bryo itself. Fig. 114 gives appearances furnislied by a vertical 
transverse section. The relations of the parts of the digestive 
tract and the mode of origin of the lungs may be learned from 
Fig. 115. 

Fio. 117.— Head of cbick of fourth day, viewed from below as an opaque object (Foster and 
Balfour). The neck is cut across between third and fourth visceral folds. C H, cerebral 
hemispheres ; F. B, vesicle of third ventricle : Op, eyeball ; n/', naso-frontal process ; m, 
cavity of mouth ; S. m, superior maxillary process of F. 1, the first visceral fold (mandibu- 
lar arch) ; F. 2, F. 3, second and third visceral arches ; N, nasal pit. 

An examination of the figures and subjoined descriptions 
must suffice to convey a general notion of the subsequent prog- 



Fig. 118.— Embryo at end of fourth day, seen as a transparent object (Foster and Balfour). 
CH, cerebral hemisphere ; F. B, fore-brain, or vesicle of third ventricle (thalamencepha- 
lon), with pineal gland (fft) projecting ; M. B, mid-brain ; Cb, cerebellum ; IV. V, fourth 
ventricle ; L, lens ; chs, choroid slit ; Cen. V, auditory vesicle ; sm., superior maxillary 
process ; li^, 22^, etc., first, second, etc. , visceral folds ; F, fifth nerve ; Fil, seventh nerve ; 
Q. Ph, glossopharynfteal nerve ; Pg, pneumogaatric. The distribution of these nerves is 
also indicated : ch. notochord ; Bt, heart ; MP, muscle-plates ; W, wing ; H. L, hind-limb. 
Tlie amnion has been removed. Al, allantois protruding from cut end of somatic stalk SS. 


ress of the embryo. Special points will be considered, either in 
a separate chapter now, or deferred for treatment in the body 
of the work from time to time, as they seem to throw light 
upon the subjects under discussion. 


This subject has been incidentally considered, but it is of 
such importance morphological, physiological, and pathological, 
as to deserve special treatment. 

In the earliest stages of the circulation of a vertebrate the 
arterial system is made up of a pair of arteries derived from the 
single bulbus arteriosus of the heart, which, after passing for- 
ward, bends round to the dorsal side of the pharynx, each giving 
off at right angles to the yelk-sac a vitelline artery ; the aortsB 
unite dorsally, then again separate and become lost in the pos- 
terior end of the embryo. The so-called arches of the aorta 
are large branches in the anterior end of the embryo derived 
from the aorta itself. 

The venous system corresponding to the above is composed 
of anterior and posterior pairs of longitudinal (cardinal) veins, 
the former (jugular, cardinal) uniting with the posterior to 
form a common trunk {ductus Cuvieri) by which the venous 
blood is returned to the heart. The blood from the posterior 
part of the yelk-sac is collected by the vitelline veins, which 
terminate in the median sinus venosus. 

The Later Stages of the Foetal Circulation. — Corresponding to 
the number of visceral arches five pairs of aortic arches arise; 
but they do not exist together, the first two having undergone 
more or less complete atrophy before the others appear. Figs. 
119, 120 convey an idea of how the permanent forms (indicated 
by darker shading) stand related to the entire system of vessels 
in different groups of animals. Thus, in birds the right (fourth) 
aortic arch only remains in connection with the aorta, the left 
forming the subclavian artery, while the reverse occurs in mam- 
mals. The fifth arch (pulmonary) always supplies the lungs. 

The arrangement of the principal vessels in the bird, mam- 
mal, etc., is represented on page 104. In mammals the two 
primitive anterior abdominal {allantoic) veins develop early 
and unite in front with the vitelline ; but the right allantoic 
vein and the right vitelline veins soon disappear, while the long 



common trunk of the allantoic and vitelline veins {ductus veno- 
sus) passes through, the liver, where it is said the ductus veno- 

FiQ. 119.— Diagrams of the aortic arches of mammal (Landois and Stirling, after Rathke). 
1. ArteriaJ trunk with one pair of arches, and an indication where the second and tliird 
pairs will develop. 2. Ideal stage of five complete arches ; the fourth clefts are shown on 
the left side. 3. The two anterior pairs of arches have disappeared. 4. Transition to the 
final stage. A, aortic arch ; ad, dorsal aorta ; ax, subclavian or axillary artery ; Ce, ex- 
ternal carotid ; Ci, internal carotid ; dB, ductus arteriosus Botalli ; P, pulmonary artery ; 
S, subclavian artery ; ta, truncus arteriosum ; v, vertebral artery. 

sus gives off and receives branches. The ductus venosus Aran- 
tii persists throughout life. (Compare the various figures illus- 
trating the circulation.) 

FiQ. 120.— Diagram illustrating transformations of aortic arches in a lizard. A ; a snake, B ; 
a bird, ; a mammal, D. Seen from below. (Haddon, after Rathke.) a, internal caro- 
tid ; b, external carotid ; c. common carotid. A. d, ductus Botalli between the third and 
fourth arches ; e, right aortic arch ; A subclavian ; g, dorsal aorta ; ft, left aortic arch ; 
i, pulmonary artery ; fc, rudiment of the ductus Botalli between the pulmonary; arteiy and 
the aortic arches. B. d, right aortic arch ; e, vertebral artery ; f, left aortic arch ; h, 
pulmonary artery ; i, ductus Botalli of the latter. C. d, origin of aorta ; e, fourth arch of 
the right side (root of dorsal aorta): /, right subclavian ; g, dorsal aorta ; ft, left subclavian 
(fourth arch ctf the left side) ; i, pulmonary artery ; k and I, right and left ductus Botalli 
of the pulmonary arteries. D. a, origin of aorta ; e, fourth arch of the left side (root of 
dorsal aorta) ; /, dorsal aorta ; g, left vertebral artery ; ft, left subclavian ; i, right sub- 
clavian (fourth arch of the right side) ; fc, right vertebral artery ; I, continuation of the 
right subclavian ; m, pulmonary artery ; n, ductus Botalli of the latter (usually termed 
ductus arfer^08^ts). 



With the development of the placenta the allantoic circula- 
tion renders the vitellinesubordinate, the vitelline and the larger 
mesenteric vein forming the portal. The portal vein at a later 
period joins one of the vencR advehentes of the allantoic vein. 

At first the vena cava inferior and the ductus venosus enter 
the heart as a common trunk. The ductus venosus Arantii 
becomes a small branch of the vena cava. 

The allantoic vein is finally represented in its degenerated 
form as a solid cord {round ligament), the entire venous sup- 
ply of the liver being derived from the portal vein. 

The development of the heart has already been traced in the 
fowl up to a certain point. In the mammal its origin and early 
progress are similar, and its further history may be gathered 
from the following series of representations. 

In the fowl the heart shows the commencement of a division 
into a right and left half on the third day, and about the 
fourth week in man, from which fact alone some idea may be 
gained as to the relative rate of development. The division 

Fia. 128. 

FiQ. 121. ■ 

Fig. 121. — Development of the heart in the human embiyo, from the fomiih to the sixth week. 
A. Embryo of four weeks (KSUiker, after Coste). B, anterior, C, posterior views of the 
heart of an embryo of six weeks (KoUiker, after Ecker). a, upper limit of buccal cavity ; 

c, buccal cavity ; b, lies between the ventral ends of second and third branchial arches ; 

d, buds of upper limbs ", I, liver ; /, intestine ; 1, superior vena cava ; 1', left superior vena 
cava : 1", opening of inferior vena cava ; 2, 2', right and left auricles ; 3, 3', right and left 
ventricles ; 4, aortic bulb. 

Fio. 122. — Human embryo of about three weeks (Allen Thomson), ur, yelk-sac; al, allantois; 
am, amnion ; ae, anterior extremity ; pe, posterior extremity. 

is effected by the outgrowth of a septum from the ventral wall, 
which rapidly reaches the dorsal side, when the double ven- 
tricle thus formed communicates by a right and left auriculo- 
ventricular opening with the large and as yet undivided auricle. 


Later an incomplete septum forms similar divisions in the auri- 
cle ; the aperture {foramen ovale) left by the imperfect growth 
of this wall persisting throughout foetal life. 

The Eustachian valve arises on the dorsal wall of the right 
auricle, between the vena cava inferior and the right and left 
vense cavEe superiores ; but in many mammals, among which is 
man, the left vena cava superior disappears during foetal life. 

For the present we may simply say that the histories of the 
development of the heart, the blood-vessels, and the blood itself 
are closely related to each other, and to the nature and changes 
of the various methods in which oxygen is supplied to the blood 
and tissues, or, in other words, to the development of the respir- 
atory system; 


Without knowing the history of the organs, the anatomical 
relations of parts with uses so unlike as reproduction on the one 
hand and excretion on the other, can not be comprehended ; nor, 
as will be shortly made clear, the fact that the same part may 
serve at one time to remove waste matters (urine) and at an- 
other the generative elements. 

The vertebrate excretory system may be divided into three 
parts, which result from the differentiation of the primitive 
kidney which has been effected during the slow and gradual 
evolution of vertebrate forms : 

1. The head-kidney (pronep Tiros). 

2. The Wolfi&an body {mesonephros). 

3. The kidney proper, or metanepTiros. 

But in this instance, as in others, to some of which allusion 
has already been made, these three parts are not functional at 
the same time. The pronephros arises from the anterior part 
of the segmental duct, pronephric. duct, duct of primitive kid- 
ney, and archinephric duct, and in the fowl is apparent on the 
third day; but the pronephros is best developed in the ich- 
thyopsida (fishes ajid amphibians). A vascular process from 
the peritoneum {glomerulus) projects into a dilated section of 
the body cavity, which is in part separated from the rest of this 
cavity {ccdom). This process, together with the segmental 
duct, now coiled, and certain short tubes developed from the 
original duct, make up the pronephros. The segmental duct 
opens at length into the cloaca. 


The mesonephros (Wolffian body), though largely developed 
in all vertebrates during foetal life, is not a persistent excretory 
organ of adult life. 

Fig. 13.3. — Diagrams illustrating development of pronephros in the fowl (Haddon). ao, aorta; 
b. c, body-cavity ; ep, epiblast with its epitrichial (flattened) layer ; hy^ hypoblast ; ni. s, 
mesoblastic somite ; n. c, neural canal ; nch, notochord ; p. t^ pronephric tubule ; so, 
somatic ; and sp, splanchnic, mesoblast. 

In the fowl recent investigation has shown that the Wolffian 
(segmental) tubes originate from outgrowths of the Wolffian 

f I- 

Fig. 124. 

Fig. 125. 

Fig. 121.— Rudimentary primitive kidney of embryonic dog. The posterior portion of the 
body of the embryo is seen from the ventral side, covered by the intestinal layer of the 
yelk-sac, which has been torn away, and thrown back in front in order to show the primi- 
tive kidney ducts with the primitive kidnej' tubes ia). 5, primitive vertebrj^e ; c, dorsal 
medulla ; d, passage into the pelvic intestinal cavity. (Haeckel, after Bischoff.) 

Fig. ViT), — Primitive kidney of a human embryo, w. the uriue-tuhes of the primitive kidney ; 
w. Wolffian duct : ir', "upper end of the latter (Blorgagni's hydatid) : /», MuUerian duct ; 
m', upper end of the latter (Fallopian hydatid) ; g, hermaphrodite gland. (After Kobelt.) 

duct and also from an intermediate cell-mass, from which lat- 
ter the Malpighian bodies take rise. The tubes, at first not con- 



nected with tlie duct, finally join it. This organ is continuous 
■with the pronephros ; in fact, all three (pronephros, mesone- 
phros, and metanephros) may be regarded as largely continua- 
tions one of another. 

The metanephros, or kidney proper, arises from mesoblast 
at the posterior part of the Wolffian body. The ureter origi- 

FiG. 126.— Section of the intermediate cell-mass of fourth day (Foster and Balfour, after Wal- 
deyer). 1 x 160. m, mesentery ; i, somatopleure ; a', portion of the germinal epitiielium 
from the duct of Miiller is formed by involution ; ti, thickened portion of the germinal 
epithelium, in which the primitive ova C and o are lying ; JSJ, modified mesoblast which 
will form the stroma of the ovary ; WK, WollHan body ; y, Wolffian duct. 

nates first from the hinder portion of the Wolffian duct. In 
the fowl the kidney tubules bud out from the ureter as rounded 
elevations. The ureter loses its connection with the Wolffian 
duct and opens independently into the cloaca. 

The following account will apply especially to the higher 
vertebrates : 

The segmental (archinephric) duct is divided horizontally 
into a dorsal or Wolffian (mesonephric) duct and a ventral or 
Miillerian duct. The Wolffian duct, as we have seen, develops 
into both ureter and kidney proper. 

To carry the subject somewhat further back, the epithelium 
lining the coelom at one region becomes differentiated into col- 
umns or cells {germinal epithelium) which by involution into 
the underlying mesoblast forms a tubule extending from before 
backward and in close relation with the Wolffian duct, thus 


forming the Miillerian duct by the process of cleavage and 
separation referred to on preceding page. 

Fig. 127.— Diagrammatic representation of the genital organs of a human embryo previous to 
sexual distinction (Allen Thomson). W, Wolffian body ; gc, genital cord ; m, MaUerian 
duct ; W, Wolffian duct ; ug^ urogenital sinus : cp, clitoris or penis ; i, intestine ; ci, 
cloaca ; 2s, part from which the scrotum or labia majora are developed ; o£, origin of tJie 
ovary or testicle respective^ ; a:, part of the Wolffian body developed later into the coni 
vasculosl ; 3, ureter ; 4, bladder ; 5, urachus. 

The future of the Miillerian and Wolffian ducts varies ac- 
cording to the sex of the embryo. 

FiQ. 128.— Diagram of the mammalian type of male sexual organs (after Qualn). Compare 
with Figs. 127, 129. C, Cowper's gland of one side ; cp, corpora cavernosa penis, cut short ; 
e, caput epididymis ; o, gubernaculum ; j, rectum ; m, hydatid of Morgagni, the persistent 
anterior end of the Miillerian duct, the conjoint posterior ends of which form the uterus 
masculinus ; ■pr., prostate gland ; s, scrotum ; sp, corpus spongiosum urethra ; f, testis 
(testicle) in the place of its original formation. The dotted line indicates the direction in 
which the testis and epididymis change place in their descent from the abdomen into the 
scrotum ; vd, vas deferens ; u/t, vas aberrans ; -us, vesicula seminalis ; W, remnants of 
Wolffian body (the organ of Girald^s or paradidymis of Waldeyer) ; 3, 4, 5, as in Fig. 189. 


In the male tlie Wolfi&an duct persists as the vas deferens ; 
in the female it remains as a rudiment in the region near the 
ovary (hydatid of Morgagni). In the female the Miillerian 
duct becomes the oviduct and related parts (uterus and vagina) ; 
in the male it atrophies. One, usually the right, also atrophies 
in female birds. The sinus pocularis of the prostate is the rem- 
nant in the male of the fused tubes. 

The various forms of the generative apparatus derived from 
the Miillerian ducts, as determined by different degrees of fu- 
sion, etc., of parts, may be learned from the accompanying 

In both sexes the most posterior portion of the Wolffian 
duct gives rise to the metanephros, or what becomes the perma- 

Fio. 129.— Diagram of the mammalian type of female sexual organs (after Quain). The dotted 
lines in one figure indicate functional drgans in the other. C, gland of Bartholin (Cowner's 
gland) ; o. c, corpus cavernosum clitoridis ; dff, remains of the left Wolffian duct, which 
may persist as the duct of Gaertner ; /, abdominal opening of left Fallopian tube ; g, 
round ligament (corresponding to the giibernaculum) ; h, hymen ; i, rectum ; I, labium ; 
m, cut Fallopian tube (oviduct, or Miillerian duct) of the right side ; n, nympha ; o, left 
ovary ; po, parovariimi ; sc, vascular bulb or corpus spongiosum ; m, uterus ; v, vulva ; 
va, vagina *, W, scattered remains of Wolffian tubes (paroophoron) ; wj, cut end of van- 
ished right Wolffian duct ; 3, ureter ; 4, bladder passing below Into the uretha; 5, urachus, 
or remnant of stalk of allantois. < 

nent kidney and ureter ; in the male also to the vas deferens, 
testicle, vas aberrans, and seminal vesicle. 

The ovary has a similar origin to the testicle ; the germinal 
epithelium furnishing the cells, which are transformed into 
Graafian follicles, ova, etc., and the mesoblast the stroma in 
which these structures are imbedded. 

In the female the parovarium remains as the representative 
of the atrophied Wolffian body and duct. 

The bladder and urachus are both remnants of the formerly 
extensive allantois. The final forms of the genito-urinary or- 


gans arise by differentiation, fusion, and atrophy: thus, the 
cloaca or common cavity of the genito-urinary ducts is divided 
by a septum (the perineum externally) into a genito-urinary 
and an intestinal (anal) part ; the penis in the male and the 
corresponding clitoris in the female appear in the region of the 
cloaca, as outgrowths which are followed by extension of folds 
of integument that become the scrotum in the one sex and the 
labia in the other. 

The urethra arises as a groove in the under surface of the 
penis, which becomes a canal. The original opening of the 
urethra was at the base of the penis. 

AL. ^ ^_ ^ 


Fig. 130. Fig. 131. 

Fig. 132. Fio. 133. 

Figs. 180 to 133. — Diagrams illustrating the evolution of the posterior passages (after Landois 
and Stirling). 

Fig. 130.— Allantois continuous with rectum. 

Fig. 131.— Cloaca formed. 

Fig. 133.— Early condition in male, before the closure of the folds of the groove on the poste- 
rior side of the penis. 

Fig. 133.— Early female condition. 

A, commencement of proctodseum ; ALL, allantois ; B, bladder ; C, penis ; CL, cloaca ; 
M, MUUerian duct ; R, rectum ; U, urethra ; S, vestibule ; SU, urogenital sinus ; V, vaa 
deferens in Fig. 133, vagina in Fig. 133. 

In certain cases development of these parts is arrested at 
various stages, from which result abnormalities frequently re- 
quiring interference by the surgeon. 

The accounts of the previous chapters do not complete the 
history of development. Certain of the remaining subjects 
that are of special interest, from a physiological point of view, 
will be referred to again; and in the mean time we shall 
consider rather briefly some of the physiological problems of 
this subject to which scant reference has as yet been made. 
Though the physiology of reproduction is introduced here, so 
that ties of natural connection may not be severed, it may 
very well be omitted by the student who is dealing with embry- 




FiQ. 134.— Various forms of mammalian uteri. A. Omlthorhynchus. B. Didelphys dorsigem. 
C. Phalangista vnlpina. D. Double uterus and vagina ; human anomaly. E.- Lepus cuni- 
culus (rabbit), uterus duplex. F. Uterus bicornis. G. Uterus bipartitus. H. Uterus 
simplex (human), a, anus ; cl^ cloaca ; o. d, oviduct ; o. t, os tincae (os uteri) ; ov, ovary ; 
r, rectum ; s, vaginal septum ; u. b, urinary bladder ; ur, ureter ; ur. o, orifice of same ; 
«s, urogenital sinus ; ut^ uterus ; u, vagina ; v. c, vaginal ceecum (Haddon). 

ology for the first time, and in any case shonld he read again 
after the other functions of the body have been studied. 

The Physiological Aspects of Development. 

According to that law of rhythm which, as we have seen, 
prevails throughout the world of animated nature, there are 
periods of growth and progress, of quietude and arrest of devel- 
opment ; and in vertebrates one of the most pronounced epochs 
— ^in fact, the most marked of all — is that by which the young , 
organism, through a series of rapid stages, attains to sexual 

While the growth and development of the generative or- 
gans share to the greatest degree in this progress, other parts of 
. the body and the entire being participate. 

So great is the change that it is common to indicate, in the 
case of the human subject, the developed organism by a new 
name— the " boy " becomes the " man," the " girl " the " woman." 
Relatively this is by far the most rapid and general of all the 
transformations the organism undergoes during its extra-uter- 
ine life. In this the entire body takes part, but very unequally. 
The increase in stature is not proportionate to the increase 
in weight, and the latter is not so great as the change in form. 
The modifications of the organism are localized and yet afEect 
the whole being. The outlines become more rounded ; the pel- 


vis in females alters in shape ; not only do the generative organs 
themselves rapidly undergo increased development, but certain 
related glands (mammae) participate; hair appears in certain 
regions of the body ; the larynx, especially in the male, under- 
goes enlargement and changes in the relative size of parts, re- 
sulting in an alteration of voice (breaking of the voice), etc. — 
all in conformity with that excess of nutritive energy which 
marks this biological epoch. 

Correlated with these physical changes are others belonging 
to the intellectual and moral (psychic) nature equally impor- 
tant, and, accordingly, the future being depends largely on the 
full and unwarped developments of these few years. 

Sexual maturity, or the capacity to furnish ripe sexual ele- 
ments (cells), is from the biological standpoint the most impor- 
tant result of the onset of that period termed, as regards the 
human species, puberty. 

The age at which this epoch is reached varies with race, 
sex, climate, and the moral influences which envelop the indi- 
vidual. In temperate regions and with European races pu- 
berty is reached at from about the thirteenth to the eighteenth 
year in the female, and rather later in the male, in whom de- 
velopment generally is somewhat slower. 

Menstruation and Ovulation. 

In all vertebrates, at periods recurring with great regu- 
larity, the generative organs of the female manifest unusual 
activity. This is characterized by increased vascularity of the 
ovary and adjacent parts; with other changes dependent on 
this, and that heightened nerve influence which, in the verte- 
brate, seems to be inseparable from all important functional 
changes. Ovulation is the maturation and discharge of ova 
from the Graafian follicles. The latter, reaching the exterior 
zone of the ovary, becoming distended and thinned, burst ex- 
ternally and thus free the ovum. The follicles being very vas- 
cular at this period, blood escapes, owing to this rupture, into 
the emptied capsule and clots ; and as a result of organization 
and subsequent degeneration undergoes a certain series of 
changes dependent on the condition of the ovary and adjacent 
parts, which varies according as the ovum has been fertilized 
or not. When fertilization occurs the Graafian follicle under- 
goes changes of a more marked and lasting character, becom- 
ing a true corpus luteum of pregnancy. 



The ovum in the fowl is fertilized in the upper part of the 
oviduct ; in the mammal mostly in this region also, as is shown 
by the site of the embryos in those groups of animals with a 
two-horned uterus, and the occasional occurrence of tubal preg- 
nancy in. woman. But this is not, in the human subject at 
least, invariably the site of impregnation. After the ovum has 
been set free, as above described, it is conveyed into the ovi- 
duct (Fallopian tube), though exactly how is still a matter of 
dispute : some holding that the current produced by the action 
of the ciliated cells of the Fallopian tube suffices ; others that 
the ovum is grasped by the fimbriated extremity of the tube as 
part of a co-ordinated act. It is likely, as in so many other 
instances, that both views are correct but partial ; that is to 
say, both these methods are employed. The columnar ciliated 
cells, lining the oviduct, act so as to produce a current in the 
direction of the uterus, thus assisting the ovum in its passage 
toward its final resting place. 

Uenstraation, — As a part of the general activity occurring 
at this time, the uterus manifests certain changes, chiefly in 
its internal mucous lining, in which thickening and increased 

Fio. 136.— Diagram of the human uterus just 
before menstruation. The shaded por- 
tion represents the mucous membrane 
(Hart and Barbour, after J. Williams). 

Fig. 136.— Uterus after menstruation has just 
ceased. The cavity of the body of the 

■ uterus is supposed to have been deprived 
of mucous membrane (J. Williams). 

vascularity are prominent. A flow of blood from the uterus 
in the form of a gentle oozing follows ; and as the superficial 


parts of the mucous lining of the uterus undergo softening 
and fatty degeneration, they are thrown off and renewed at 
these periods {catamenia, menses, etc.), provided pregnancy 
does not take place. In mammals helow man, in their nat- 
ural state, pregnancy does almost invariably take place at 
such times, hence this exalted activity of the mucous coat of 
the uterus, in preparation for the reception and nutrition of 
the ovum, is not often in vain. In the human subject the 
menses appear monthly ; pregnancy may or may not occur, and 
consequently there may be waste of nature's forces; though 
there is a certain amount of evidence that menstruation does 
not wholly represent a loss ; but that it is largely of that char- 
acter among a certain class of women is only too evident. As 
can be readily understood, the catamenial flow may take place 
prior to, during, or after the rupture of the egg-capsule. 

As the uterus is well supplied with glands, during this 
period of increased functional activity of its lining membrane, 
mucus in considerable excess over the usual quantity is dis- 
charged ; and this phase of activity is continued should preg- 
nancy occur. 

All the parts of \h.e generative organs are supplied with 
muscular tissue, and with nerves as well as blood-vessels, so 
that it is possible to understand how, by the influence of nerve- 
centers, the various events of ovulation, menstruation, and 
those that follow when pregnancy takes place, form a related 
series, very regular in their succession, though little prominent 
in the consciousness of the individual animal wten normal. 

The Nutrition of the Ovum (oosperm). 

This will be best understood if it be remembered that the 
ovum is a cell, undifferentiated in most directions, and thus a 
sort of amoeboid organism. In the fowl it is known that the 
cells of the primitive germ devour, amcsba-like, the yelk-cells, 
while in the mammalian oviduct the ovum is surrounded by 
abundance of proteid, which is doubtless utilized in a somewhat 
similar fashion, as also in the uterus itself, until the embryonic 
membranes have formed. To speak of the ovum being nour- 
ished by diffusion, and especially by osmosis, is an unnecessary 
assumption, and, as we believe, at variance with fundamental 
principles ; for we doubt much whether any vital process is 
one of pure osmosis. As soon as the yelk-sac and allantois 
have been formed, nutriment is derived in great part through 


the vessel- walls, -wMcli, it will be remembered, are differentia- 
ted from the cells of the mesoblast, and, it may well be as- 
sumed, have not at this early stage entirely lost their amoeboid 
character. The blood-vessels certainly have a respiratory func- 
tion, and suffice, till the more complicated villi are formed. 
The latter structures are in the main similar in build to the 
villi of the alimentary tract, and are adapted to being sur- 
rounded by similar structures of maternal origin. Both the 
maternal crypts and the fcetal villi are, though complementary 
in shape, all biit identical in minute structure in most in- 
stances. In each case the blood-vessels are covered superfi- 
cially by cells which we can not help thinking are essential in 
nutrition. The villi are both nutritive and respiratory. It is 
no more difficult to understand their function than that of the 
cells of the endoderm of a polyp, or the epithelial coverings of 
lungs or gills. 

Experiment proves that there is a respiratory interchange 
of gases between the maternal and fcetal blood which nowhere 
mingle physically. The same law holds in the respiration of 
the foetus as in the mammals. Oxygen passes to the region 
where there is least of it, and likewise carbonic anhydride. If 
the mother be asphyxiated so is the foetus, and indeed more 
rapidly than if its own umbilical vessels be tied, for the mater- 
nal blood in the first instance abstracts the oxygen from that 
of the foetus when the tension of this gas becomes lower in the 
maternal than in the foetal blood ; the usual course of affairs 
is reversed, and the mother satisfies the oxygen hunger of her 
own blood and tissues by withdrawing that which she recently 
supplied to the foetus. It will be seen, then, that the embryo is 
from the first a parasite. This explains that exhaustion which 
pregnancy, and especially a series of gestations, entails. True, 
nature usually for the time meets the demand by an excess of 
nutritive energy : hence many persons are never so vigorous in 
appearance as when in this condition ; often, however, to be fol- 
lowed by corresponding emaciation and senescence. The full 
and frequent respirations, the bounding pulse, are succeeded by 
reverse conditions ; action and reaction are alike present in the 
animate and inanimate worlds. Moreover, it falls to the parent 
to eliminate not only the waste of its own organism but that of 
the foetus ; and not infrequently in the human subject the over- 
wrought excretory organs, especially the kidneys, fail, entailing 
disastrous consequences. 

The digestive functions of the embryo are naturally inact- 


ive, tlie blood being supplied witli all its needful constituents 
tbrougli tlie placenta by a much shorter process ; indeed, the 
placental nutritive functions, so far as the foetus is concerned, 
may be compared with the removal of already digested material 
from the alimentary canal, though of course only in a general 
way. During foetal life the digestive glands are developing, 
and at the time of birth all the digestive juices are secreted in 
an efficient condition, though only relatively so, necessitating a 
special liqufd food (milk) in a form in which all the constituents 
of a normal diet are provided, easy of digestion. 

Fia. 137. — Human g:erms or embryos from the second to the fifteenth week (uatm'al size), seen 
from the left side, the arched baolc turned toward the right. (Princlpaily after Ecker.) 
n, human embryo of 14 days ; m, of 3 weelra ; JV, of 4 weeks ; V, of 5 weeks ; VI, of 6 
weeks ; vn, of 7 weeks ; vm, of 8 weeks ; XH, of 13 weeks ; XV, of 15 weeks. 

Bile, inspissated and mixed with the dead and cast-off epi- 
thelium of the alimentary tract, is abundant in the intestine at 
birth in the human subject ; but bile is to be regarded perhaps 
rather in the light of an excretion than as a digestive fluid. 
The skin and kidneys, though not functionless, are rendered 
unnecessary in great part by the fact that waste can be and is 
withdrawn by the placenta, which proves to be a nutritive, re- 


spiratory, and excretory organ ; it is in itself a sort of abstract 
and brief chronicle of the whole physiological story in foetal life. 

All of the foetal organs, especially the muscles, abound in an 
animal starch (glycogen), which in some way, not well under- 
stood, forms a reserve fund of nutritive energy which is pretty 
well used up in the earlier months of pregnancy. We may 
suppose that the amoeboid cells — all the undifferentiated cells 
of the body — feed on it in primitive fashion ; and it will not 
be forgotten that the older the cells become, the "more do they 
depart from the simpler habits of their earlier, cruder existence ; 
hence the disappearance of this substance in the later months 
of foetal life. 

In one respect the foetus closely resembles the adult: it 
draws the pabulum for all its various tissues from blood which 
itself may be regarded as the first completed tissue. We are, 
accordingly, led to inquire how this river of life is distributed ; 
in a word, into the nature of the foetal circulation. 

Foetal Circulation. — The blood leaves the placenta by the um- 
bilical vein, reaches the inferior vena cava, either ditectly (by 
the ductus venosus), or, after first passing to the liver (by the 
vencR advehentes, and returning by the vence. revehentes), and 
proceeds, mingled with the blood returning from the lower ex- 
tremities, to the right auricle. This blood, though far from 
being as arterial in character as the blood after birth, is the 
best that reaches the heart or any part of the organism. After 
arriving at the right auricle, being dammed back by the Eus- 
tachian valve, it avoids the right ventricle, and shoots on into 
the left auricle, passing thence into the left ventricle, from 
which it is sent into the aorta, and is then carried by the great 
trunks of this arch to the head and upper extremities. The 
blood returning from these parts passes into the right auricle, 
then to the corresponding ventricle and thence into the pul- 
monary artery; but, finding the branches of this vessel un- 
opened, it takes the line of least resistance through the ductus 
arteriosus into the aortic arch beyond the point where its great 
branches emerge. It will be seen that the blood going to the 
head and upper parts of the body is greatly more valuable as 
nutritive pabulum than the rest, especially in the quantity of 
oxygen it contains ; that the blood of the foetus, at best, is rela- 
tively ill-supplied with this vital essential ; and as a result we 
find the upper (anterior in quadrupeds) parts of the foetus best 
developed, and a decided resemblance between the mammalian 
foetus functionally and the adult forms of reptiles and kindred 


groups of the lower vertebrates. But this condition is well 
enough adapted to the general ends to be attained at this pe- 

Pulmonaty Art 

Foramen Ovale, 

Eitsiachian Valve. 

Right Auric. -Vent. Opening. 

Hepatic Vein. 

Branches of (he 
Umbilical Vein, 
to the lAver. 


Pidmonary Art. 
Left Auricle. 
...Left Auric. • Vent. 

~" Dvuitua Venogu$. 

Internal Iliac Arteries. 
Fig. 138. —Diagram of the foetal circulation (Flint). 


riod — the nourisliment of structures on the way to a higher 
path of progress. 

As emhryonic maturity is being reached, preparation is made 
for a new form of existence ; so it is found that the Eustachian 
valve is less prominent and the foramen ovale smaller. 


All the efforts that have hitherto been made to determine 
the exact cause of the result of that series of events which make 
up parturition have failed. This has probably been owing to 
an attempt at too simple a solution. The foetus lies surrounded 
(protected) by fluid contained in the amniotic sac. For its expul- 
sion there is required, on the one hand, a dilatation of the uter- 
ine opening {os uteri), and, on the other, a vis a tergo. The lat- 
ter is furnished by the contractions of the uterus itself, aided by 
the simultaneous action of the abdominal miiscles. Through- 
out the greater part of gestation the uterus experiences some- 
what rhythmical contractions, feeble as compared with the 
final ones which lead to expulsion of the foetus, but to be regard- 
ed as of the same character. With the growth and functional 
development of other organs, the placenta becomes of less con- 
sequence, and a fatty degeneration sets in, most marked at the 
periphery, usually where it is thinnest and of least use. It does 
not seem rational to believe that the onset of labor is referable 
to any one cause, as has been so often taught ; but rather that it 
is the final issue to a series of processes long existing and grad- 
ually, though at last rapidly, reaching that climax which seems 
like a vital storm. The law of rhythm affects the nervous sys- 
tem as others, and upon this depends the direction and co-ordi- 
nation of those many activities which make up parturition. 
We have seen that throughout the whole of foetal life changes 
in one part are accompanied by corresponding changes in oth- 
ers ; and in the final chapter of this history it is not to be ex- 
pected that this connection should be severed, though it is not 
at present possible to give the evolution of this process with 
any more than a general approach to probable correctness. 

Changes in the Circulation after Birth. 

When the new-born mammal takes the first breath, effected 
by the harmonious action of the respiratory muscles, excited 
to action by stimuli reaching them from the nerve-center (or 


centers) wMcli preside over respiration, owing to its being 
roused into action by the lack of its accustomed supply of 
oxygen, the hitherto solid lungs are expanded ; the pulmonary 
vessels are rendered permeable, hence the blood now takes the 
path of least resistance along them, as it formerly did through 
the ductus arteriosus. The latter, from lack of use, atrophies 
in most instances. The blood, returning to the left auricle of 
the heart from the lungs in increased volume, so raises the 
pressure in this chamber that the stream that formerly flowed 
through the foramen ovale from the right auricle is opposed 
by a force equal to its own, if not greater, and hence passes by 
an easier route into the right ventricle. The fold that tends to 
close the foramen ovale grows gradually over the latter, so that 
it usually ceases to exist in a few days after birth. 

At birth, ligature of the umbilical cord cuts off the placental 
circulation ; hence the ductus venosus atrophies and becomes a 
mere ligament. 

The placenta, being now a foreign body in the uterus, is ex- 
pelled, and this organ, by the contractions of its walls, closes the 
ruptured and gaping vessels, thus providing against haemor- 

Coitus between the Sexes. 

In all the higher vertebrates congress of the sexes is essential 
to bring the male sexual product into contact with the ovum. 

Fig. 139.— Section of erectile tissue (Cadiat). o, trabeculee of connective tissue, with elastic 
fibers, and bundles of plain muscular tissue (c) ; &, venous spaces (Schaf er). 


Erection of the penis results from the conveyance of an 
excess of blood to the organ, owing to dilation of its arteries, 
and the retention of this blood within its caverns. 

The structure of the penis is peculiar, and, for the details of 
the anatomy of both the male and female generative organs, 
the student is referred to works on this subject ; suffice it to 
say that it consists of erectile tissue, the chief characteristic of 
which is the opening of the capillaries into cavernous venous 
spaces (sinuses) from which the veinlets arise ; with such an 
arrangment the circulation must be very slow — the inflow 
being greatly in excess of the outflow — apart altogether from 
the compressive action of certain muscles connected with the 
organ. As previously explained, the spermatozoa originate in 
the seminal tubes, from which they find their way to the 

Fio. 140.— Section of parts of three seminiferous tubules of tlie rat (Schafer). a, with the 
spermatozoa least advanced in development ; 6, more advanced ; c, containing fully de- 
veloped spermatozoa. Between the tubules are seen strands of interstitial cells, with 
blood-vessels and lymph-spaces. 

seminal vesicles or receptacles for semen till required to be 
discharged. The spermatozoa as they mature are forced on by 
fresh additions from behind and by the action of the ciliated 
cells of the epididymis, together with the wave-like (peristaltic) 
action of the vas deferens. Discharge of semen during coitus 
is effected by more vigorous peristaltic action of the vas defer- 
ens and the seminal vesicles, followed by a similar rhythmical 
action of the bulbo-cavernosus and ischio-cavernosus muscles, 
by which the fluid is forcibly ejaculated. 

Semen itself, though composed essentially of spermatozoa, 


is mixed with the secretions of the vas deferens, of the seminal 
vesicles, of Cowper's glands, and of the prostate. Chemically 
it is neutral or alkaline in reaction, highly albuminous, and 
contains nuclein, lecithen, cholesterin, fats, and salts. 

The movements of the male cell, owing to the action of the 
tail (cilium), suffice of themselves to convey them to the ovi- 

FiG. 141. — Left broad ligament, Fallopian tube, ovary, and parovarium in the human subject 
(Henle). u, uterus ; i, isthmus of Fallopian tube ; a, ampulla ; /, fimbriated end of the 
tube, with the parovarium to its right ; o. ovary ; o. I, ovarian ligament. 

ducts ; but there is little doubt that during or after sexual con- 
gress there is in the female, even in the human subject, at least 

Fig. 142.— Uterus and ovaries of the sow, semi-diagrammatic {after Dalton). o, ovary ; if. 
Fallopian tube ; A, horn of the uterus ; 6, body of the uterus ; v, vagina. 

in many cases, a retrograde peristalsis of the uterus and ovi- 
ducts which would tend to overcome the results of the activity 


of the ciliated cells lining the oviduct. It is known that the 
male cell can survive in the female organs of generation for 
several days, a fact not difficult to understand, from the method 
of nutrition of the female cell (ovum) ; for we may suppose 
that both elements are not a little alike, as they are both 
slightly modified amoeboid organisms. 

Nervous Mechanism. — Incidental reference has been made to 
the directing influence of the nervous system over the events 
of reproduction ; especially their subordination one to another 
to bring about the general result. These may now be consid- 
ered in greater detail. 

Most of the processes in which the nervous system t^kes 
part are of the nature of reflexes, or the result of the automa- 
ticity (independent action) of the nerve-centers, increased by 
some afferent (ingoing) impressions along a nerve-path. It is 
not always possible to estimate the exact share each factor 
takes, which must be highly variable. Certain experiments 
have assisted in making the matter clear. It has been found 
that, if in a female dog, the spinal cord be divided when the 
animal is still a puppy, menstruation and impregnation may 
occur. If the same experiment be performed on a male dog, 
erection of the penis and ejaculation of semen may be caused 
by stimulation of the penis. As the section of the cord has left 
the hinder part of the animal's body severed from the brain, 
the creature is, of course, unconscious of anything happening 
in all the parts below the section, of whatever nature. If the 
nervi erigentes (from the lower part of the spinal cord) be 
stimulated, the penis is erected ; and if they be cut, this act be- 
comes impossible, either reflexly by experiment or otherwise. 
Seminal emissions, it is well known, may occur during sleep, 
and may be associated, either as result or cause, with voluptu- 
ous dreams. Putting all these facts together, it seems reason- 
able to conclude that the lower part of the spinal cord contains 
the nervous machinery requisite to initiate those influences (im- 
pulses) which, passing along the nerves to the generative or- 
gans, excite and regulate the processes which take place in 
them. In these, vascular changes, as we have seen, always 
play a prominent part. 

Usually we can recognize some afferent influence, either 
from the brain (psychical), from the surface — at all events 
from without that part of the nervous system (center) which 
functions directly in the various sexual processes. It is com- 
mon to speak of a number of sexual centers — as the erection 


center, the ejaculatory center, etc. — ^bntwe mucli doubt whether 
there is snch sharp division of physiological lahor as these 
terms imply, and they are liable to lead to misconception ; ac- 
cordingly, in the present state of our knowledge, we prefer to 
speak of the sexual center, using even that term in a somewhat 
broad sense. 

The effects of stimulation of the sexual organs are not con- 
fined to the parts themselves, but the ingoing impulses set up 
radiating outgoing ones, which affect widely remote areas of 
the body, as is evident, especially in the vascular changes ; the 
central current of nerve influence breaks up into many streams 
as a result of the rapid and extensive rise of the outflowing 
current, which breaks over ordinary barriers, and takes paths 
which are not properly its own. Bearing this fact in mind, 
the chemical composition of semen, so rich in proteid and other 
material valuable from a nutritive point of view, and consid- 
ering how the sexual appetites may engross the mind, it is not 
difficult to understand that nothing so quickly disorganizes the 
whole man, physical, mental, and moral, as sexual excesses, 
whether by the use of the organs in a natural way, or from 

Nature has protected the lower animals by the strong bar- 
rier of instinct, so that habitual sexual excess is with them an 
impossibility, since the females do not permit of the approaches 
of the male except during the rutting period, which occurs only 
at stated, comparatively distant periods in most of the higher 
mammals. When man keeps his sexual functions in subjection 
to his higher nature, they likewise tend to advance his whole 

Summary. — Certain changes, commencing with the ripening • 
of ova, followed by their discharge and conveyance into the 
uterus, accompanied by simultaneous and subsequent modifica- 
tions of the uterine mucous membrane, constitute, when preg- 
nancy occurs, an unbroken chain of biological events, though 
usually described separately for the sake of convenience. 
When impregnation does not result, there is a retrogression in 
the uterus (menstruation) and a return to general quiescence 
in all the reproductive organs. 

Parturition is to be regarded as the climax of a variety of 
rhythmic occurrences which have been gradually gathering 
head for a long period. The changes which take place in the 
placenta of a degenerative character fit it for being cast off, and 
may render this structure to some extent a foreign body before 


it and the foetus are finally expelled, so that these changes may- 
constitute one of a number of exciting causes of the increased 
uterine action of parturition. But it is important to regard the 
whole of the occurrences of pregnancy as a connected series of 
processes co-ordinated by the central nervous system so as to 
accomplish one great end, the development of a new individual. 

The nutrition of the ovum in its earliest stages is effected by 
means in harmony with its nature as an amoeboid organism ; 
nutrition by the cells of blood-vessels is similar, while that by 
villi may be compared to what takes place through the agency 
of similar structures in the alimentary canal of the adult mam- 

The circulation of the foetus puts it on a par physiologically 
with the lower vertebrates. Before birth there is a gradual 
though somewhat rapid preparation, resulting in chginges 
which speedily culminate after birth on the establishment of 
the permanent condition of the circulation of extra-uterine life. 

The blood of the foetus (as in the adult) is the great store- 
house of nutriment and the common receptacle of all waste 
products ; these latter are in the main transferred to the moth- 
er's blood indirectly in the placenta ; in a similar way nutri- 
ment is imported from the mother's blood to that of the foetus. 
The placenta takes the place of digestive, respiratory, and ex- 
cretory organs. 

Coitus is essential to bring the male and female elements 
together in the higher vertebrates. The erection of the penis is 
owing to vascular changes taking place in an organ composed 
of erectile tissue ; ejaculation of semen is the result of the peri- 
staltic action of the various parts of the sexual tract, aided by 
.rhythmical action of certain striped muscles. The spermatozoa, 
which are unicellular, flagellated (ciliated) cells, make up the es- 
sential part of semen ; though the latter is complicated by the 
addition of the secretions of several glands in connection with 
the seminal tract. Though competent by their own movements 
of reaching the ovum in the oviduct, it is probable that the 
uterus and oviduct experience peristaltic actions in a direction 
toward the ovary, at least in a number of mammals. 

The lower part of the spinal cord is the seat in the higher . 
mammals of a sexual center or collection of cells that receives 
afferent impulses and sends out efferent impiilses to the sexual 
organs. This, like all the lower centers, is under the control of 
the higher centers in the brain, so that its action may be either 
initiated or inhibited by the cerebrum. 



The study of reproduction has prepared the student for the 
comprehension of certain views of the origin of the forms of 
life which could not be as profitably considered before. 

"While the great majority of biologists are convinced that 
there has been a gradual evolution of more complex organisms 
from simpler ones, and while most believe that Darwin's the- 
ory furnishes some of the elements of a solution of the problem 
as to how this has occurred, many still feel that the whole ex- 
planation was not furnished by that great naturalist. 

Accordingly, we shall notice very briefly a few of the more 
important contributions to this subject since Darwin's views 
were published. 

In America, under the influence of the writings of Cope and 
Hyatt, a school of evolutionists has been formed, holding doc- 
trines that constitute a modification of those announced in 
cruder form by Lamarck, hence termed neo-Lamarckianism. 
These authors have imported consciousness into the list of 
factors of organic evolution and given it a prominent place. 
They regard consciousness as a fundamental property of proto- 
plasm ; it determines effort and the direction that activity shall 
take : thus hunger leads to migration, and brings the creature 
under a new set of conditions which influence its nature. A 
certain proportion of the changes an animal undergoes are at- 
tributed to the direct influence of surrounding conditions (en- 
vironment), but the larger number are owing to efforts involv- 
ing the greater use of some parts than others, which tends to 
become habitual. This is the explanation neo-Lamarckianism 
offers for the origin of variations, It is assumed that the re- 
sults of use or disuse of parts is inherited, so that the gain or 
loss is not transient with the individual, but remains with the 

This theory also refers the loss or preservation of certain 
structures to " acceleration " or " retardation " of growth ; thus, 
if the growth of gills were greatly and progressively retarded 
during embryonic life, they might become only rudimentary, 
and this would furnish an explanation of the origin of rudiment- 
ary organs, though it is clear that use and effort could not di- 
rectly explain such acceleration or retardation. It is further a 
fact, which this theory does not explain, that all variations of 
structure produced by use are not inherited. 


Weismann, in fact, denies that peculiarities acquired dur- 
ing the lifetime of the adult are passed on to offspring. This 
writer believes that we must seek in Amoeha, as the ancestral 
representative of the ovum, for the clew to the laws of heredity. 
The Amoeba must divide or cease to exist as a group form — 
hence the segmentation of the ovum ; this is but the inherited 
tendency to divide. What the individual becomes is determined 
entirely by the ovum, the whole of which does not develop into 
the new being, but a part is laid aside in reserve as the future 
ovum. Any variations that show themselves in future indi- 
viduals are such as arise from the variations of the ovum itself. 

According to this writer, it is as natural for the offspring to 
resemble the parent (heredity) in the higher groups of animals 
as that one Amceba should resemble another, and for the same 

Weismann has also attempted to explain the necessity and 
the significance of the extrusion of polar globules. The first 
polar globule is expelled from all ova, even those that can de- 
velop independent of a male cell (parthenogenetic). This rep- 
resents that part of the original ovum which determines its 
peculiarities of form, etc. (ovogenetic idioplasm) ; while the 
second polar globule is one half of the nucleus of the mature 
ovum ready to enter upon development, if fertilized. When 
the latter takes place, it is joined by the corresponding nuclear 
substance of the male cell to form the segmentation nucleus. 
It is this substance (germ-plasma) which determines exactly 
what line of development, to the minutest details, the ovum 
shall follow. In the course of time the nucleus would thus 
come to represent many generations of united plasmas. There 
must be a limit to this, from the physical necessities of the case ; 
hence the expulsion of a second polar globule, which also is a 
provision against parthenogenesis, for in some cases the plasma 
of the nucleus has the power, without the accession of any 
male plasma, to segment and develop the mature animal. But 
in any case there is a great advantage in the union of the two 
plasmas with their diverse experiences; hence sexual repro- 
duction, though the most costly apparently, is in reality the 
most economical for Nature in the end, for higher results are 
reached, and it seems, in fact, that this lies at the very foun- 
dation of organic progress. 

The theory of Brooks may be regarded as eclectic, being a 
combination of that of Weismann and Darwin more particu- 
larly, with entirely new additions by himself. 


Darwin believed that every part of the body gave off " gem- 
mules," or very minute bodies, which were collected into the 
ovum, and thus the ovum came to be a sort of abstract of the 
whole body — hence the resemblance of offspring to parents, . 
since the development of the ovum was but that of the gem- 
mules. Some of the gemmules might remain latent for genera- 
tions, and then develop; hence that resemblance often seen to 
ancestors more remote than the parents (reversion). This is a 
very brief account of Darwin's hypothesis of pangenesis. 

This writer, however, never accounted for variations. He 
spoke of variations as " spontaneous," meaning, not that they 
were supernatural, but that it was not possible to assign them 
to a definite cause. To account for variation has naturally been 
the aim of later writers. How neo-Lamarckianism does this 
has been already considered. We now give the views of Brooks 
on this and other points in connection with organic evolution. 

This thinker, like Weismann, looks to the fertilized ovum 
for an explanation of the main facts ; but Brooks refers the 
origin of variations to the influence of the male cell. This is, 
of course, a pure hypothesis, but it is in harmony with many 
facts which were in need of explanation. It had been noticed 
by Darwin that variations of all kinds were most apt to arise 
upon alteration in the conditions under which an animal 
lived. Brooks also believes in gemmules, but does not think 
they are. given off from all parts equally or at all times, but 
that they are derived from those parts most affected by the 
change of surroundings ; and since this would influence parts 
much when for the worse, variation would coincide with suf- 
fering or need ; hence those very parts would vary, and so pre- 
pare for adaptation, just when this was most called for by the 
nature of the case. But the male sexual element, it has been 
shown, is more liable to variation than the ovum ; hence the ex- 
planation of what Brooks believes to be a fact, that it is the 
sperm-cell that generally is responsible for variation, since it 
chiefly collects the gemmules. 

The author of this theory points to parthenogenetic forms 
being less variable, as evidence of the truth of his view. To 
introduce a male cell is to impart vast numbers of new gem- 
mules, and thus induce variability. This hypothesis would ex- 
plain why the female represents what is most fundamental and 
ancient in the history of psychological development, and the 
male what is associated with enterprise^n a word, the female 
preserves, the male originates, in the widest sense. 



Vines has stated that the equivalent of parthenogenesis 
takes place in the male cell in plants. . Though this may be an 
objection to the universality of application of Brooks's theory, 
it does not seem to us to be fatal to it as a whole. 

As has been pointed out, in a previous chapter, Darwin held 
that the differences that caused ultimately the formation of 
new groups of living forms were the result of extremely slow 
accumulation of variations, at first very minute. He every- 
where insists upon this. But, unquestionably, it is just here 
that the greatest difiBculty is to be encountered in the Darwin- 
ian account of evolution. The chances ag'ainst the loss of the 
variation by breeding with forms that did not possess it seem 
to be numerous, hence various theories have been proposed to 
lessen the difficulty. 

Mivart introduced the doctrine of extraordinary births, be- 
lieving that variations were often sudden and pronounced. 
That they were so occasionally Darwin himself admitted ; but 
he considered a theory like that of Mivart as a surrender, a 
resort to an explanation that verged in its character on the 
introduction of the supernatural itself. 

A view that has attracted much attention and caused a 
great deal of controversy, is that of Romanes, which was intro- 
duced in part to meet the difficulty just referred to ; and to lessen 
the further one arising from the infertility of species with one 
another, as compared with the perfect fertility of varieties. It 
has often been noticed that, though the difference anatomically 
between varieties might be greater than between species, the 
above law as to fertility still held. Such a fact calls for ex- 
planation ; hence Romanes has proposed his theory of " physi- 
ological selection" (segregation, isolation). If it be admitted 
that some change may take place in the sexual organs of two 
forms so that the members of one are fertile with each other 
while those of the other are not, it will at once appear that 
they are as much isolated physiologically as if separated by an 
ocean. That such does take place is an assumption based on 
the great tendency in the reproductive organs to change ; and 
it is claimed that, if this assumption be granted, that the main 
difficulty of Darwin's theory will be removed, for the " swamp- 
ing " action of intercrossing forms that vary slightly, or one of 
them not at all, in the given direction, will not occur. Romanes 
believes that forms that vary are fertile inter se, but not with 
the parent forms, which would meet the case fairly well. Cer- 
tain it is that species are not generally fertile with one an- 


other while varieties are so invariably ; and it is this that, in 
the opinion of Romanes and many others, has never been ade- 
quately explained. 

Admitting that the theories of Eomanes, Brooks, and Weis- 
mann have advanced us on the way to more complete views of 
the mode of origin of the forms of the organic world, it must 
still be felt that all theories yet propounded fall short of being 
entirely satisfactory. It seems to us unfortunate that the sub- 
ject has not received more attention from physiologists, as 
without doubt the final solution must come through that sci- 
ence which deals with the properties rather than the forms 
of protoplasm ; or, in other words, the fundamental principles 
underlying organic evolution are physiological. But, in the 
unraveling of a subject of such extreme complexity, all sci- 
ences must probably contribute their quota to make up the 
truth, as many rays of different colors compounded form white 
light. As with other theories of the inductive sciences, none 
can be more than temporary ; there must be constant modifi- 
cation to meet increasing knowledge. Conscious that any 
views we ourselves advance must sooner or later be modified 
as all others, even if acceptable now, we venture to lay before 
the reader the opinions we have formed upon this subject as 
the result of considerable thought. 

All vital phenomena may be regarded as the resultant of 
the action of external conditions and internal tendencies. Amid 
the constant change which life involves we recognize two 
things : the tendency to retain old modes of behavior, and the 
tendency to modification or variation. Since those impulses 
originally bestowed on matter when it became living, must, in 
order to prevail against the forces from without, which tend ' 
to destroy it, have considerable potency, the tendency to modi- 
fication is naturally and necessarily less than to permanence of 
form and function. 

From these principles it follows that when an Amoeba or 
kindred organism divides after a longer or shorter period, it is 
not in reality the same in all respects as when its existence 
began, though we may be quite unable to detect the changes ; 
and when two infusorians conjugate, the one brings to the other 
protoplasm different in molecular behavior, of necessity, from 
having had different experiences. We attach great importance 
to these principles, as they seem to us to lie at the root of the 
whole matter. What has been said of these lower but inde- 


pendent forms of life applies to tlie liiglier. All organisms are 
made up of cells or aggregations of cells and their products. 
For the present we may disregard the latter. . When a muscle- 
cell by division gives rise to a new cell, the latter is not identi- 
cally the same in every particular as the parent cell was origi- 
nally. It is what its parent has become by virtue of those 
experiences it has had as a muscle-cell per se, and as a member 
of a populous biological community, of the complexities of 
which we can scarcely conceive. 

Now, as a body at rest may remain so, or may move in a 
certain direction according as the forces acting upon it exactly 
counterbalance one another, or produce a resultant efPect in 
the direction in which the body moves, so in the case of he- 
redity, whether a certain quality in the parent appears in the 
offspring, depends on whether this quality is neutralized, aug- 
mented, or otherwise modified by any corresponding quality in 
the other parent, or by some opposite quality, taken in connec- 
tion with the direct influence of the environment during devel- 

This assumption explains among other things why acquired 
peculiarities (the results of accident, habit, etc.) may or may 
not be inherited. 

These are not usually inherited because, as is to be expect- 
ed, those forces of the organism which have been gathering 
head for ages are naturally not easily turned aside. Again, we 
urge, heredity must be more pronounced than variation. 

The ovum and sperm-cell, like all other cells of the body, 
are microcosms representing the whole to a certain extent in 
themselves — that is to say, cell A is what it is by reason of what 
all the other millions of its fellows in the biological republic 
' are ; so that it is possible to understand why sexual cells repre- 
sent, embody, and repeat the whole biological story, though it 
is not yet possible to indicate exactly how they more than 
others have this power. This falls under the laws of speciali- 
zation and the physiological division of labor ; but along what 
paths they have reached this we can not determine. 

Strong evidence is furnished for the above views by the his- 
tory of disease. Scar-tissue, for example, continues to repro- 
duc6-itself as such; like produces like, though in this instance 
the like is in the first instance a departure from the normal. 
Gout is well known to be a hereditary disease ; not only so, but 
it arises in the offspring at about the same age as in the parent, 
which is equivalent to saying that in the rhythmical life of 


certain cells a period is reached -when they display the behav- 
ior, physiologically, of their parents. Yet gout is a disease 
that can be traced to peculiar habits of living and may be 
eventually escaped by radical changes in this respect — ^that is 
to say, the behavior of the cells leading to gout can be induced 
and can be altered ; gout is hereditary, yet eradicable. 

Just as gout may be set up by formation of certain modes 
of action of the cells of the body, so may a mode of behavior, in 
the nervous system, for example, become organized or fixed, be- 
come a habit, and so be transmitted to offspring. It will pass 
to the descendants or not according to the principles already 
noticed. If so fixed in the individual in which it arises as to 
predominate over more ancient methods of cell behavior, and 
not neutralized by the strength of the normal physiological ac- 
tion of the corresponding parts in the other parent, it will reap- 
pear. We can never determine whether this is so or not before- 
hand ; hence the fact that it is impossible, especially in the case 
of man, whose vital processes are so modified by his psychic 
life, to predict whether acquired variations shall become heredi- 
tary ; hence also the irregularity which characterizes heredity 
in such cases ; they may reappear in offspring or they may not. 
In viewing heredity and modification it is impossible to get a 
true insight into the matter without taking into the account 
both original natural tendencies of living matter and the influ- 
ence of environment. We only know of vital manifestations 
in some environment ; and, so far as our experience goes, life is 
impossible apart from the influence of surroundings. With 
these general principles to guide us, we shall attempt a brief 
examination of the leading theories of organic evolution. 

First of all, Spencer seems to be correct in regarding evolu- 
tion as universal, and organic evolution but one part of a 
whole. No one who looks at the facts presented in every field 
of nature can doubt that struggle (opposition, action and reac- 
tion) is universal, and that in the organic world the fittest to a 
given environment survives. But Darwin has probably fixed 
his attention too closely on this principle and attempted to ex- 
plain too much by it, as well as failed to see that there are 
other deeper facts underlying it. Variation, which this author 
scarcely attempted to explain, seems to us to be the natural re- 
sult of the very conditions under which living things have an 
existence. Stable equilibrium is an idea incompatible with our 
fundamental conceptions of life. Altered function implies al- 
tered molecular action, which sometimes leads to appreciable 


structural cliange. From our conceptions of the nature of liv- 
ing matter, it naturally follows that variation should be great- 
est, as has been observed, under the greatest alteration in the 

We are but very imperfectly acquainted as yet with the 
conditions under which life existed in the earlier epochs of the 
earth's history. Of late, deep-sea soundings and arctic explo- 
rations have brought surprising facts to light, showing that 
living matter can exist under a greater variety of conditions 
than was previously supposed. Thus it turns out that light is 
not an essential for. life everywhere. We think these recent 
revelations of unexpected facts should make us cautious in as- 
suming that life always manifested itself under conditions 
closely similar to those we know. Variation may at one period 
have been more sudden and marked than Darwin supposes; 
and there does seem to be room for such a conception as the 
" extraordinary births " of Mivart implies ; though we would not 
have it understood that we think Darwin's view of slow modi- 
fication inadequate to produce a new species ; we simply vent- 
ure to think that he was not justified in insisting so strongly 
that this was the only method of Nature ; or, to put it more 
.justly for the great author of the " Origin of Species," with the 
facts that have accumulated since his time he would scarcely 
be warranted in maintaining so rigidily his conviction that 
new forms arose almost exclusively by the slow process he has 
so ably described. 

As there must be all degrees in consciousness, we do not 
deny that it may be logical to assume some dim spark of this 
quality in all protoplasm, as Cope insists ; and that it plays a 
part in determining action and growth there seems to be no 
doubt. But is it not more philosophical to regard conscious- 
ness and all allied qualities as correlatives, and underlaid by a 
molecular constitution with which it is associated as other qual- 
ities ? It is unduly exalted in the neo-Lamarckian philosophy. 

We must allow a great deal to use and effort, doubtless, and 
they explain the origin of variations up to a certain point, but 
the solution is only partial. Variations must arise as we have 
attempted to explain, and use and disuse are only two of the 
factors amid many. Correlated growth, or the changes in one 
part induced by changes in another, is a principle which, though 
recognized by Dairwin, Cope, and others, has not, we think, re- 
ceived the attention it deserves. To the mind of the physiolo- 
gist, all changes must be correlated with others. 


This principle lias played a great part in the development of 
man, as we shall show later. 

Weismann's theories have called attention to the ovum in a 
new and valuable way, though he seems to have given too ex- 
clusive attention to the nucleus {germ-plasma) in itself and 
out of relation to the influence of the countless cells that 
make up the hody and must be constantly determining modi- 
fications of the generative organs and the sexual cells them- 
selves ; so that Brooks's explanation, by adding a new factor, 
or, at least, presenting a new aspect of the case, was called 
for and seems to be warranted on the general principle that 
advance in protoplasmic life is dependent on new experiences, 
and that the male cell represents a little world of the concen- 
trated experiences gathered during the lifetime of the or- 
ganism that produced it. But we must consider the whole 
doctrine of gemmules as a crude and entirely unnecessary 

In what sense has the line th^t evolution has taken been 
predetermined ? In the sense that all things in the universe 
are unstable, are undergoing change, leading to new forms and 
qualities of such a character that they result in a gradual prog- 
ress toward what our minds can not but consider higher mani-. 
f estations of being. 

The secondary methods according to which this takes place 
constitute the laws of nature, and as we learn from the progress 
of science are very numerous. The unity of nature is a real- 
ity toward which our conceptions are constantly leading us. 
Evolution is a necessity of living matter (indeed, all matter) as 
we view it. 


One visiting the ruins of a vast and elaborate building, 
which had been thoroughly pulled to pieces, would get an 
amount of information relative to the original structure and 
uses of the various parts of the edifice largely in proportion to 
his familiarity with architecture and the various trades which 
make that art a practical success. The study of the chemistry 
of the animal body is illustrated by such a case. Any attempt 
to determine the exact chemical composition of living matter 
inust result in its destruction ; and the amount of information 
conveyed by the examination of the chemical ruins, so to speak. 


will depend a great deal on the knowledge already possessed of 
ctemical and vital processes. 

It is in all probability true that the nature of any vital pro- 
cess is at all events closely bound up with the chemical changes 
involved ; but we must not go too far in this direction. We are 
not yet prepared to say that life is only the manifestation of 
certain chemical and physical processes, meaning thereby such 
chemistry and physics as are known to us ; nor are we prepared 
to go the length of those who regard life as but the equivalent 
of some other force or forces ; as electricity may be considered 
as the transformed representative of sp much heat and vice 
versa. It may be so, but we do not consider that this view is 
warranted in the present state of our knowledge. 

On the other hand, vital phenomena, when our investiga- 
tions are pushed far enough, always seem to be closely asso- 
ciated with chemical action ; hence the importance to the stu- 
dent of physiology of a sound knowledge of chemical princi- 
ples. We think the most satisfactory method of studying the 
functions of an organ will be found to be that which "takes into 
consideration the totality of the operations of which it is the 
seat, together with its structure and chemical composition; 
hence we shall treat chemical details in the chapters devoted to 
special physiology, and here give only such an outline as will 
bring before the view the chemical composition of the body in 
its main outlines ; and even many of these will gather a signifi- 
cance, as the study of physiology progresses, that they can not 
possibly have at the present. 

Fewer than one third of the chemical elements enter into 
the composition of the mammalian body ; in fact, the great 
bulk of the organism is composed of carbon, hydrogen, nitro- 
gen, and oxygen; sodium, potassium, magnesium, calcium, 
sulphur, phosphorus, chlorine, iron, fluorine, silicon, though 
occurring in very small quantity, seem to be indispensable to 
the living body ; while certain others are evidently only pres- 
ent as foreign bodies or impurities to be thrown out sooner 
or later. It need scarcely be said that the elements do not 
occur as such in the living body, but in combination form- 
ing salts, which latter are usually united with albuminous 
compounds. As previously mentioned, the various parts which 
make up the entire body of an animal are composed of living 
matter in very different degrees ; hence we find in such parts 
as the bones abundance of salts, relative to the proportion of 
proteid matter ; a condition demanded by that rigidity without 


whicli an internal skeleton would be useless, a defect well illus- 
trated by that disease of the bones known as rickets, in which 
the lime-salts are insufficient. It is manifest that there may be 
a very great variety of classifications of the compounds found 
in the animal body according as we regard it from a chemical, 
physical, or physiological point of view, or combine many 
aspects in one whole. The latter is, of course, the most correct 
and profitable method, and as such is impossible at this stage 
of the student's progress ; we shall simply present him with the 
following outline, which will be found both simple and com- 
prehensive.* The subject of Animal Chemistry will be found 
treated in detail in the Appendix. 


Such food as supplies energy directly must contain carbon 

Living matter or protoplasm always contains nitrogenous 
carbon compounds. 

In consequence, C, H, 0, N, are the elements found in great- 
est abundance in the body. 

The elements S and P are associated with the nitrogenous 
carbon compounds ; they also form metallic sulphates and phos- 

CI and F form salts with the alkaline metals Na, K, and the 
earthy metals Ca and Mg. 

Fe is found in hcemoglobin and its derivatives. 

Protoplasm, when submitted to chemical examination, is 
killed. It is then found to consist of proteids, fats, carbohy- 
drates, salines, and extractives. 

It is probable that when living it has a very complex mole^ 
cule consisting of C, H, O, N, S, and P chiefly. 


1 A • i ^^^ Nitrogenous. | ggrtain crystalline bodies. 

1. Organic. ■< , carbohydrates. 

( (b) Non-nitrogenous, j p , ■' 

„ _ .1 Mineral salts. 

2. Inorganic. I ^^^^ 

Salts. — In general, the salts of sodium are more characteris- 
tic of ammaL tissues and those of potassium of vegetable tissues. 

* Taken from the author's " Outlines of Lectures on Physiology," W. Drysdale 
& Co., Montreal. 


Na 01 is more abundant in the fluids of animals ; K and 
phosphates more abundant in the tissues. 

Earthy salts are most abundant in the harder tissues. 

The salts are probably not much, if at all, changed in their 
passage through the body. 

In some cases there is a change from acid to neutral or 

The salts are essential to preserve the balance of the nutri- 
tive processes. Their absence leads to disease, e. g., scurvy. 


They are the chief constituents of most living tissues, in- 
cluding blood and lymph. 

The molecule consists of a great number of atoms (complex 
constitution), and is formed of the elements C, H, N, O, S, and P. 

All proteids are amorphous. 

All are non-diffusible, the peptones excepted. 

They are soluble in strong acids and alkalies, with change 
of properties or constitution. 

In general, they are coagulated by alcohol, ether, and heating. 

Coagulated proteids are soluble only in strong acids and 

Classification and Distinguishing Characters of Proteids. 

1. Native albumins : Serum albumin ; egg albumin ; soluble 
in water. 

2. Derived albumins (albuminates) : Acid and alkali albu- 
min ; casein ; soluble in dilute acids and alkalies, insoluble in 
water. Not precipitated by boiling. 

3. Globulins : Globulin (globin) ; paraglobulin ; myosin ; 
fibrinogen. Soluble in dilute saline solutions, and precipitated 
by stronger saline solutions. 

4. Peptones: Soluble in water; diffusible through animal 
membranes ; not precipitated by acids, alkalies, or heat. De- 
rived from the digestion (peptic, pancreatic) of all proteids. 

5. Fibrin: Insoluble in water and dilute saline solutions. 
Soluble, but not readily, in strong saline solutions and in dilute 
acids and alkalies. 


The following bodies are allied to proteids, bift are not the 
equivalents of the latter in the food. 


They are all composed of C, H, N, 0. Cliondrin, gelatin, 
ceratin have, in addition, S. 

Chondrin: The organic basis of cartilage. Its solutions 
set into a firm jelly on cooling. 

Gelafdn : The organic basis of bone, teeth, tendon, etc. Its 
solutions set (glue) on cooling. 

Elastin : The basis of elastic tissue. Its solutions do not set 
jelly-like (gelatinize). 

Mucin : From the secretion of mucous membranes ; precipi- 
tated by acetic acid, and insoluble in excess. 

Keratin : Derived from hair, nails, epidermis, horn, feathers. 
Highly insoluble. 

Nuclein: Derived from the nuclei of cells. Not digested 
by pepsin ; contains P but no S. 


The fats are hydrocarbons ; are less oxidized than the carbo- 
hydrates ; are inflammable ; possess latent energy in a high 

Chemically, the neutral fats are glycerides or ethers of the 
fatty acids, i. e., the acid radicles of the fatty acids of the oleic 
and acetic series replace the exchangeable atoms of H in the 
triatomic alcohol glycerine, e. g. : 

Glycerine. Palmitic acid. Glycerine tripalmitate or palmitin. 

) OH H0.0C.C„H3, ( O.CO.CH,, 

OaHs [ OH + HO.OC.CsHa, = CsHs ] O.CO.C^H,, + 3H»0 
) OH HO.OC.CsHa, ( O.CO.CisHai 

A soap is formed by the action of caustic alkalies on 
fats, e. g. : 

Tripalmitin, Potassium palmitate. 

(cSb.) } O. + 8 (KOH, = 3 I («-H.O) [ + '^* I 0. 

The soap may be decomposed by a strong acid into a fatty 
acid and glycerine, e. g. : 

CaHai.CO.K -f- HCl = C,5H3,.CO,H -f- KCl. 

Potassium palmitate. Palmitic acid. 

The /ate are insoluble in water, but soluble in hot alcohol, 
ether, chlorofbrm, etc. 

The alkaline soaps are soluble in water. 


Most animal fats are mixtures of several kinds in varying 
proportion ; hence the melting-point for the fat of each species 
of animal is different. 


Lecithin, Protdgon, Cerebrin : 

They consist of C, H, N, O, and the first two of P in addi- 

They occur in the nervous tissues. 


General formula. Cm (HjO)„. 

1. The Sugars : Dextrose, or grape-sugar, CeHiaOj + HjO 
readily undergoes alcoholic fermentation; less readily lactic 

Lactose, milk-sugar, CisHjaOn + HsO ; susceptible of the lactic 
acid fermentation. 

Inosit, or muscle-sugar, CeHisOe + 3HsO ; capable of the lac- 
tic fermentation. 

Maltose, CisHjjOn + HsO, capable of the alcoholic fermenta- 
tion. The chief sugar of the digestive process. 

All the above are much less sweet and soluble than ordinary 

3. The Starches : Glycogen, CsHioOs, convertible into dex- 
trose. Occurs abundantly in many fcetal tissues and in the 
liver, especially of the adult animal. 

Dextrin, CsHioOs, convertible into dextrose. Soluble in 
water ; intermediate between starch and dextrose ; a product 
of digestion. 

Pathological: Grape-sugar occurs in the urine in diabetes 

Certain substances formed within the body may be regarded 
as chiefly waste-products, the result of metabolism or tissue- 

They are divisible into nitrogenous metabolites and non- 
nitrogenous metabolites. 

Nitrogenous Metabolites. 

1. Urea, uric acid and compounds, kreatinin, xanthin, hypo- 
xanthin (sarkin), hippuric acid, all occurring in urine. 

2. Leucin, tyrosin, taurocholic, and glycocholic acids, which 
occur in the digestive tract. 


3. Kreatin, constantly found in muscle, and a few others of 
less constant occurrence. 

The above consists of C, H, N, 0. Taurocholic acid contains 
also S. 

The molecule in most instances is complex. 

Non-Nitrogenous Metabolites. 

These occur in small quantity, and some of them are secreted 
in an altered form. 

They include lactic and sarcolactic acid, oxalic acid, succinic 
acid, etc. 


We propose in this chapter to examine into the methods 
employed in physiological investigation and teaching, and the 
character of conclusions arrived at by physiologists as depend- 
ent on a certain method of reasoning. 

The first step toward a legitimate conclusion in any one of 
the inductive sciences to which physiology belongs is the col- 
lection of facts which are to constitute the foundation on 
which the inference is to be based. If there be any error in 
these, a correct conclusion can not be drawn by any reliable 
logical process. On the other hand, facts may abound in thou- 
sands and yet the correct conclusion never be reached, because 
the method of interpretation is faulty, which is equivalent to 
saying that the process of inference is either incomplete or in- 
correct. The conclusions of the ancients in regard to nature 
were usually faulty from errors in both these directions ; they 
neither had the requisite facts, nor did they correctly interpret 
those with which they were conversant. 

Let us first examine into the methods employed by modern 
physiologists, and determine in how far they are reliable. First, 
there is the method of direct observation, in which no appara- 
tus whatever or only the simplest kind is employed ; thus, the 
student may count his own respirations, feel his own heart- 
beats, count his pulse, and do a very great deal more that will 
be pointed out hereafter ; or he may examine in like manner an- 
other fellow-being or one of the lower animals. This method 
is simple, easy of application, and is that usually employed by 
the physician even at the present day, especially in private 


practice. The value of the results obviously depends on the 
reliability of the observer in two respects : First, as to the ac- 
curacy, extent, and delicacy of his perceptions ; and, secondly, 
on the inferences based on these sense-observations. Much 
must depend on practice — that is to say, the education of the 
senses. The hand may become a most delicate instrument of 
observation ; the eye may learn to see what it once could not ; 
the ear to detect and discriminate what is quite beyond the 
uncultured hearing of the many. But it is one of the most 
convincing evidences of man's superiority that in every 
field of observation he has risen above the lower animals> 
some of which by their unaided senses naturally excel him. 
So in this science, instruments have opened up mines of facts 
that must have otherwise remained hidden; they have, as 
it were, provided man with additional senses, so much have 
the natural powers of those he already possessed been sharp- 

But the chief value of the results reached by instruments 
consists in the fact that the movements of the living body can 
be registered ; i. e., the great characteristic of modern physiol- 
ogy is the extensive employment of the graphic method, which 
has been most largely developed by the distinguished French 
experimenter Marey. Usually the movements of the point of 
lever are impressed on a smoked surface, either of glazed 
paper or glass, and rendered permanent by a coating of some 
material applied in solution and drying quickly, as shellac in 
alcohol. The surface on which the tracing is written may be 
stationary, though this is rarely the case, as the object is to get 
a succession of records for comparison ; hence the most used 
form of writing surface is a cylinder which may be raised or 
lowered, and which is moved around regularly by some sort of 
clock-work. It follows that the lever-point, which is moved by 
the physiological effect, describes curves of varying complexity. 
That tracings of this or any other character should be of any 
value for the purposes of physiology, they must be susceptible 
of relative measurement both for time and space. This can be 
accomplished only when there is a known base-line or abscissa 
from which the lever begins its rise, and a time record which is 
usually in seconds or portions of a second. The first is easily 
obtained by simply allowing the lever to write a straight line 
before the physiological effect proper is recorded. Time inter- 
vals are usually indicated by the interruptions of an electric 
current, or by the vibrations of a tuning-fork, a pen or writer 


of some kind being in eacli instance attaclied to the apparatus 
so as to record its movements. 

As levers, in proportion to their length, exaggerate all the 
movements imparted to them, a constant process of correction 
must be carried on in the mind in reading the records of the 
graphic method, as in interpreting the field of view presented 
by the microscope. 

The student is especially warned to carry on this process, 
otherwise highly distorted views of the reality will become 
fixed in his own mind ; and certainly a condition of ignorance 
is to be preferred to such false knowledge as this may become. 
But it is likewise apparent that movements that would without 
such mechanism be quite unrecognized may be rendered visible 
and utilized for inference. There is another source of possible 
misconception in the use of the graphic method. The lever is 
• sometimes used to record the movements of a column of fluid 
(manometer. Fig. 307), as water or mercury, the inertia of which 
is considerable, so that the record is not that of the lever as 
affected by the physiological (tissue) movement, but that move- 
ment conveyed through a fluid of the kind indicated. Again, 
all points, however delicate, write with some friction, and the 
question always arises, In how far is that friction sufficient to 
be a source of inaccuracy in the record ? When organs are di- 
rectly connected with levers or apparatus in mechanical rela- 
tion with them, one must be sure that the natural action of the 
organ under investigation is in no way modified by this con- 

From these remarks it will be obvious that in the graphic 
method physiologists possess a means of investigation at once 
valuable and liable to mislead. Already electricity has been 
extensively used in the researches of physiologists, and it is to 
this and the employment of photography that we look in the 
near future for methods that are less open to the objections we 
have noticed. 

However important the methods of physiology, the results 
are vastly more so. We next notice, then, the progress from 
methods and observations to inferences, which we shall en- 
deavor to make clear by certain cases of a hypothetical charac- 
ter. Proceeding from the brain and entering the substance of 
the heart, there is in vertebrates a nerve known as the vagus. 
Suppose that, on stimulating this nerve by electricity in a rab- 
bit, the heart ceases to beat, what is the legitimate inference ? 
Apparently that the effect has been due to the action of the 


nerve on the heart, an action excited by the use of electricity. 
This does not, however, according to the principles of a rigid 
logic, follow. The heart may have ceased beating from some 
cause wholly unconnected with this experiment, or from the 
electric current escaping along the nerve and affecting some 
nervous mechanism within the heart, which is not a part of the 
vagus nerve ; or it may have been due to the action of the cur- 
rent on the muscular tissue of the heart directly, or in some other 
way. But suppose that invariably, whenever this experiment 
is repeated, the one result (arrest of the beat) follows, then it is 
clear that the vagus nerve is in some way a factor in the causa- 
tion. Now, if it could be ascertained that certain branches of 
the nerve were distributed to the heart-muscle directly, and that 
stimulation of these gave rise to arrest of the cardiac pulsation, 
then would it be highly probable, though not certain, that there 
was in the first instance no intermediate mechanism; while- 
this inference would become still more probable if in hearts 
totally without any such nervous apparatus whatever, such a 
result followed on stimulation of the vagus. Suppose, further, 
that the application of some drug or poison to the heart pro- 
vided with special nervous elements besides the vagus termi- 
nals prevented the effect before noticed on stimulating the 
vagus, while a like result followed under similar circumstances 
in those forms of heart unprovided with such nervous struct- 
ures, there would be additional evidence in favor of the view 
that the result we are considering was due solely to some action 
of the vagus nerve ; while, if arrest of the heart followed in the 
first case but not in the second, and this result were invariable, 
there would be roused the suspicion that the action of the 
vagus was not direct, but through the nervous structures with- 
in the heart other than vagus endings. And if, again, there were 
a portion of the rabbit's heart to which there were distributed 
this intrinsic nervous supply, which on stimulation directly 
was arrested in its pulsation, it would be still more probable 
that the effect in the first instance we have considered was due 
to these structures, and only indirectly to the vagus. But be it 
observed, in all these cases there is only probability. The con- 
clusions of physiology never rise above probability, though this 
may be so strong as to be practically equal in value to absolute 
certainty. Would it be correct, from any or all the experi- 
ments we have supposed to have been made, to assert that the 
vagus was the arresting (inhibitory) nerve of the heart ? All 
hearts thus far examined have much in common in structure 


and function, and in so far is the above generalization probable. 
Such, a statement would, however, be far from that degree of 
probability which is possible, and should therefore not be ac- 
cepted till more evidence has been gathered. The mere resem- 
blance in form and general function does not suffice to meet the 
demands of a critical logic. Such a statement as the above would 
not necessarily apply to the hearts of all vertebrates or even all 
rabbits, if the experiments had been conducted on one animal 
alone, for the result might be owing to a mere idiosyncrasy of 
the rabbit under observation. The further we depart from the 
group of animals to which the creature under experiment be- 
longs, the less is the probability that our generalizations for 
the one class will apply to another. It will, therefore, be seen 
that wide generalizations can not be made with that amount of 
certainty which is attainable until experiments shall have be- 
come very numerous and widely extended. A really broad and 
sound physiology can only be constructed when this science 
has become much more comparative — ^that is, extended to many 
more groups and sub-groups of animals than at present. 

To attempt to generalize for the heart, the kidney, the liver, 
etc., when only the dog, cat, rabbit, and frog, have been made 
as a rule the subjects of experiment, except for the groups of 
animals to which the above belong, is not only hazardous but 
positively illogical ; while to denominate conclusions based on 
such experiments, even when supplemented by the teachings 
of disease, " human physiology " is, in the writer's opinion, a 
wholly unwarrantable proceeding. 

It is this conviction which has had much to do with this 
book being written; to the introduction of the comparative 
element ; and the separation so frequently in form as well as 
in reality of facts and inferences. A genuine human physi- 
ology, with the exact nature and value of the inferences clearly 
stated, is yet to be written ; and it seems not only judicious, 
but demanded as a matter of candor and honesty, to state at 
the outset to the student what we feel able to teach confidently, 
and what must be presented as feebly probable or barely pos- 

Human physiology proper must of necessity be accumulated 
slowly. Much may be, indeed must be, inferred from the ex- 
periments disease is making ; still, certain forms of accident or 
surgical operation provide the opportunity to investigate the 
human body in. health or in a moderately near approach to that 
condition. Close self-observation under a variety of condi- 



tions, so precisely defined as to meet the demands of science, 
may be made by the intelligent student. Much of this might 
be verified in the case of other healthy persons. Some of it is 
in certain respects of more value than any experiments that 
can be made upon the lower animals, for the latter can not 
communicate to us their sensations; in their case all our in- 
formation must be derived from the use of our own senses, 
mostly unaided by any reports of theirs. 

It is not possible during any experiment, especially any one 
in which vivisection is employed, to observe the animal under 
conditions that are strictly normal, for, by the very nature of 
the case, we have rendered it abnormal. We must in all such 
instances draw conclusions with corresponding caution. It 
will be understood that the expression "conclusive experi- 
ment," as applied to such a case, is only approximately correct. 

At the present time it is very common to experiment upon 
organs disconnected, either anatomically or physiologically 
(functionally), from the rest of the body to a greater or less 
extent. This is termed the isolated method. It has the advan- 
tage of being more simple, and permits of the study of certain 
points apart from others — one factor being considered inde- 
pendently of the rest in the physiological total. But, in draw- 
ing conclusions, it is very important in such a case not to forget 
the premises. There is manifest danger of making the gener- 
alization wider than the facts warrant. It is only when such 
experiments are supplemented by a great many others, and 
when judged in connection with the action of the organ under 
consideration, as it is influenced by other organs, that such re- 
sults can be of great value in building up a normal physiology. 
To know, for example, that the isolated heart behaves in a cer- 
tain manner is not useless information, but its value depends 
entirely on the conclusions drawn from it, especially as to what 
it is conceived as teaching of the functions of the heart as it 
beats within the body of an animal while it walks, or flies, or 
swims, in carrying out the purpose of its being. 

We have incidentally alluded to the teaching of disease. 
" Disease " is but a name for disordered function. One viewing 
a piece of machinery for the first time in improper action might 
draw conclusions with comparative safety, provided he had a 
knowledge of the correct action of similar machines. Our ex- 
perience gives us a certain knowledge of the functions of our 
own bodies. By ordinary observation and by experiment on 
other animals we get additional data, which, taken with the 


disordered action resulting from gross or molecular injury 
(disease), gives a basis for certain conclusions as to the normal 
functions of the human body or those of lower animals. This 
information is especially valuable in the case of man, since he 
can report with a fair degree of reliability, in most diseased 
conditions, his own sensations. 

It is hoped that this brief treatment of the methods and 
logic of physiology will suffice for the present. Throughout 
the work they will be illustrated in every chapter, though not 
always with distinct references to the nature of the intellectual 
process followed. 

Summary. — There are two methods of physiological observa- 
tion, the direct and the indirect. The first is the simplest, and 
is valuable in proportion to the accuracy and, delicacy and 
range of the observer ; the latter implies the use of apparatus, 
and is more complex, more extended, more delicate, and precise. 
It is usually employed with the graphic method, which has the 
advantage of recording and thus preserving movements which 
correspond with more or less exactness to the movements of 
tissues or organs. It is valuable, but liable to errors in record- 
ing and in interpretation. 

The logic of physiology is that of the inductive sciences. It 
proceeds from the special to the general. The conclusions of 
physiology never pass beyond extreme probability, which, in 
some cases, is practically equal to certainty. It is especially 
important not to make generalizations that are too wide. 


It is a matter of common observation that the loss of the 
whole, or a very large part, of the blood of the body entails 
death ; while an abundant haemorrhage, or blood-disease in any 
of its forms, causes great general weakness. 

The student of embryology is led to inquire as to the neces- 
sity for the very early appearance and the rapid development 
of the blood- vascular system so prominent in all vertebrates. 

An examination of the means of transit of the blood, as 
already intimated, reveals a complicated system of tubes dis- 
tributed to every organ and tissue of the body. These facts 
would lead one to suppose that the blood must have a tran- 
scendent importance in the economy, and such, upon the most 
minute investigation, proves to be the case. The blood has 



been aptly compared to an internal world for tlie tissues, an- 
swering to the external world for tlie organism as a whole. 
This fluid is the great storehouse containing all that the most 
exacting cell can demand ; and, further, is the temporary 
receptacle of all the waste that the most busy cell requires to 
discharge. Should such a life-stream cease to flow, the whole 
vital machinery must stop — death must ensue. 

Comparative. — It will prove more scientific and generally sat- 
isfactory to regard the blood as a tissue having a fluid and 
flowing matrix, in which float cellular elements or corpuscles — 
a view of the subject that is less startling when it is remem- 
bered that the greater part of the protoplasm which makes up 
the other tissues of the body is of a semifluid consistence. In 
all animals possessing blood, the matrix is a clear, usually more 
or less colored fluid. Among invertebrates the color may be 
pronounced: thus, in cephalopods and some crustaceans it is 
blue, but in most groups of animals and all vertebrates the 
matrix is either colorless or more commonly of some slight 
tinge of yellow. Invertebrates with few exceptions possess 
only colorless corpuscles, but all vertebrates have colored cells 
which invariably outnumber the other variety, and display 

forms and sizes which 
are sufi&ciently constant 
to be characteristic. In 
all groups below mam- 
mals the colored corpus- 
cles are oval, mostly bi- 
convex, and nucleated 
during all periods of the 
animal's existence ; in 
mammals they are cir- 
cular biconcave disks 
(except in the *amel 
tribe, the j^orpuscles of 
which are oval), and in 
post-embryonic life with-~ 
out a nucleus ; nor do 
they possess a cell-wall. 
The red cells vary in size 
in different groups and 
sub-groups of animals, being smaller the higher the place the 
animal occupies, as a general rule : thus, they are very large 
in vertebrates below mammals, in some cases being almost 

Fig. 143.— Leucocytes of human blood, showing amoe- 
boid movements (Landois). These movements are 
not normaUy in the blood-vessels so marked as pic- 
tured here, so that the figure represents an ex- 
treme case. 



Fig. 144. — Photograph of colored corpuscles of 
frog. 1 X 370. (Alter Hint.) 

visible to the unaided eye, while in the whole class of mam- 
mals they are very minute ; their numbers also in this group 
are vastly greater than in 
others lower in the scale. 

The average size in man 
is -j^TT inch ("0077 mm.) and 
the number in a cubic mil- 
limetre of the blood about 
5,000,000 for the male and 
500,000 less for the female, 
which would furnish about 
250,000,000,000 in a pound 
of bloodi It will be under- 
stood that averages only are 
spoken of, as all kinds of 
variations occur, some of 
which will be referred to 
later, and their significance 

Under the microscope the blood of vertebrates is seen to 
owe its color to the cells chiefly, and, so far as the red goes, 

almost wholly. Corpuscles 
when seen singly are never 
of the deep red, however, 
of the blood as a whole, 
but rather a yellowish red, 
the tinge varying some- 
what with the class of ani- 
mals from which the spec- 
imen has been taken. 

Certain other morpJio- 
logical elements found in 
mammalian blood deserve 
brief mention, though their 
significance is as.yet a mat- 
ter of much dispute : 

1. The blood - plates 
{plaques, JuBmatoblasts, 
third element), very small, 
colorless, biconcave disks, which are deposited in great num- 
bers on any thread or similar foreign body introduced into the 
circulation, and rapidly break up when blood is shed. 

2. On a slide of blood that has been prepared for some little 

Fig. 14B.— Corpuscles from human subject (Funke). 
A few colorless corpuscles are seen among the 
colored disks, which are many of them arranged 
in rouleaux. 



time, aggregations of very minute granules {elementary gran- 
ules) may be seen. These are supposed to represent the disin- 
tegrating protoplasm of the corpuscles. 


Fig. 146.— Blood-plaques and their derivatives (Landois, after Bizzozero and Laker). 1, red 
blood-corpuscles on the flat ; 2, from the side ; 3, unchanged blood-plaques ; 4, lymph- 
corpuscle surrounded with blood-plaques ; S, blood-plaques variously altered ; 6, lymph- 
corpuscle with two masses of fused blood-plaques and threads of fibrin ; 7, group of 
blood-plaques fused or run together ; 8, similar small mass of partially dissolved blood- 
plaques with fibrils of fibrin. 

The pale or colorless corpuscles are very few in number in 
mammals compared with the red, there being on the average 
only about 1 in 400 to 600, though they become much more 
numerous after a meal. They are granular in appearance, and 
possess one or more nuclei, which are not, however, readily seen 
in all cases without the use of reagents. They are character- 
ized by greater size, a globular form, the lack of pigment, and 
the tendency to amcsboid movements, which latter may be ex- 
aggerated in disordered conditions of the blood, or when the 
blood is withdrawn and observed under artificial conditions. 
It will be understood that these cells (leucocytes) are not con- 
fined to the blood, but abound in lymph and other fluids. 
They are the representatives of the primitive cells of the em- 
bryo, as is shown by their tendency (like ova) to throw out 
processes, develop into higher forms, etc. In behavior they 
strongly suggest Amoe,ba and kindred forms. 

We may, then, say that in all invertebrates the blood, when 
it exists, consists of a plasma {liquor sanguinis), in which float 
the cellular elements which are colorless; and that in verte- 
brates in addition there are colored cells which are always nu- 
cleated at some period of their existence. The colorless cells 



are globular masses of protoplasm, containing one or more 
nuclei, and "witli tlie general character of amoeboid organisms. 


The History of the Blood-Cells. 

We have already seen that the blood and the vessels in 
which it flows have a common origin in the mesoblastic cells of 
the embryo chick ; the . same applies to mammals and lower 
groups. The main facts may be grouped under two head- 
ings: 1. Development of the blood-corpuscles during embry- 
onic life. 3. Development of the corpuscles in post-embryonic 

In the bird and the mammal, cells of the mesoblast in the 
area opaca give off processes which unite ; later they become 
hollowed out {vacuolated), 
and thus form capillaries. 
At the same time the nuclei 
of these cells multiply {pro- 
liferate), gather small por- 
tions of the protoplasm of 
the main cells about them, 
become colored, and thus 
form the nucleated corpus- 
cles of- the embryo. This, 
or a similar process, is known 
to occur in some animals 
{rat) after birth ; but in the 
human foetus there is a grad- 
ual decline in the number of 
nucleated cells found free in 

the blood, and at birth they Fig 147.-SurfaceviewfrombeIowof asmallpor- 
' •' tionof posterior end of pellucid area of a cmck 

are very rare, which is prob- -- — - ■ .„„,„-_. ._. 

ably the case with most 


While the origin of the 
red cells, as above described, may be regarded as the earliest 
and most general, it is not their exclusive source. 

When the liver has been formed this organ seems to carry 
on a development begun in the spleen, for the nucleated but as 
yet colorless cells formed in the spleen seem to become pig- 
mented in the liver. 

There is also evidence that colored corpuscles may arise by 
endogenous formation in the lymphatic glands. 

ty-six hours, 1 x 400 (Foster and Bal- 
0. c, .blood-corpuscles ; a, nuclei, which 

of thirty 

four). . , 

subsequently become nuclei of cells forming 
walls of blood-vessels ; p. pr. protoplasmic 
processes, containing nuclei with large nu- 
cleoli, n. 



There is no doubt that the greater number of the non-nucle- 
ated corpuscles are derived from the nucleated forms. 

The post-embryonic development of colored corpuscles is 
naturally less understood from the greater difficulties attend- 

c ^ 

Fig. 148. 

(3a O 

Fig. 149. 

Fig. 151. 

Fig. 160. 



Fig. 153. 

Fig. 148.— Cell elements of red marrow, a, large granular marrow cells ; 6, smaller, more 
vesicular cells ; c, free nuclei, or small lympnold cells, some of which may be even sur- 
rounded with a delicate rim of protoplasm ; d, nucleated red corpuscles of the bone 

Fig. 149.— Nucleated red cells of marrow, illustrating mode of development into the ordinaiy 
non-nucleated red corpuscles, a, common forms of the colored nucleated cells of red mar- 
row ; 6, 1, 2, 3, gradual disappearance of the nucleus ; c, large non-nucleated red corpuscle 
resembling 3 and 3 of b, in all respects save in the absence of any trace of nucleus. 

Fig. 150. — Nucleated red corpuscles, illustrating the migration of the nucleus from the cell, a 
process not unfrequently seen. in the red marrow. 

Fig. 151.— Blood of embryo of four months, a, 1, 3, 3, 4, nucleated red corpuscles. In 4 the 
same granular disintegrated appearance of the nucleus as is noted in marrow cells, b, 1, 
microcyte ; 3, raegalocyte ; 3, ordinary red corpuscle. 

Fig. 153.— From spleen. 1, blood-plaques, colorless and varying a little in size ; 2, two micro- 
cytes of a deep-red color ; 3, two ordinary red corpuscles ; 4, a solid, translucent, lymphoid 
cell or free nucleus. (Figs. 148-152, after Osier.) 

ing its investigation. The following may be regarded as a 
summary of the chief facts or rather opinions on this subject : 

1. From the colorless cells ; though, whether the nucleus 
disappears, or remains to form the chief part of the cell and 
become pigmented, is undetermined. 

2. From peculiar cells of the red marrow of the bones (head, 
trunk, etc.), though there is also some doubt as to whether the 


nuclei of these cells remain or not ; but as all grades of transi- 
tion forms have been found in the bone-marrow ; since anaemia 
occurs in disease of bones; since the bone-marrow has been 
found in an unusually active condition after hsemorrhage and 
under other circumstances demanding a rapid replacement of 
lost cells — there seems to be little room for doubt that in the 
adult the red marrow of the bones is the chief site of the devel- 
opment of red corpuscles. It is not, however, the only one, for 
under peculiar stress of need even the lymphatic glands pro- 
duce red cells, and the latter have been seen to be budded off 
from the spleen in a young animal (kid). 

The colorless cells of the blood first arise as migrated undif- 
ferentiated remnants of the early embryonic cell colonies. That 
they remain such is seen by their physiological behavior, to be 
considered a little later. Afterward they are chiefly J)roduced 
from a peculiar form of connective tissue known as leucocy- 
tenic, and which is gathered into organs, the chief function of 
which (lymphatic glands) is to produce these cells, though this 
tissue is rather widely distributed in the mammalian body in 
other forms than these. 

Summary. — The student may, with considerable cert^-inty, 
consider the colorless corpuscle of the blood as the most primi- 
tive ; the red, derived either from the white or some form of 
more specialized cell ; the nucleated, as the earlier and more 
youthful form of the colored corpuscle, which may in some 
groups of vertebrates be replaced by a more specialized (or de- 
graded ?) non-nucleated form mostly derived directly from the 
former ; that in the first instance the blood-vessels and blood 
arise simultaneously in the mesoblastic embryonic tissue ; that 
such aii origin may exist after birth, either normally in some 
mammals or under unusual functional need ; that the red 
marrow is the chief birthplace of colored cells in adult life ; 
that the spleen, liver, lymphatic glands, and other tissues of 
similar structure contribute in a less degree to the develop- 
ment of the red corpuscles ; and that the last mentioned organs 
are the chief producers of the colorless amoeboid blood-cells. 

Finally, it is well to remember that Nature's resources in 
this, as in many other cases, are numerous, and that her mode 
of procedure is not invariable ; and that, if qne road to an end is 
blocked, another is taken. 

The Decline and Death of the Blood-Cells. — The blood-corpuscles, 
like other cells, have a limited duration, with the usual chapters 
in a biological history of rise, maturity, and decay. There is 


reason to believe that the red cells do not live longer than a 
few weeks at most. The red cells, in various degrees of disor- 
ganization, have heen seen within the white cells {phagocytes), 
and the related cells of the spleen, liver, bone-marrow, etc. In 
fact, these cells, by virtue of retained ancestral {amczboid) quali- 
ties, have devoured the weakened, dying red cells. It seems to 
be a case of survival of the fittest. It is further known that 
abundance of pigment containing iron is found in both spleen 
and liver ; and there seems to be no good reason for doubting 
that the various pigments of the secretions of the body [urine, 
hile, etc.) are derived from the universal pigment of the blood. 
These coloring matters, then, are to be regarded as the excreta 
in the first instance of cells behaving like amoeboids, and later 
as the elaborations of certain others in the kidney and else- 
where, the special function of which is to get rid of waste 
products. The birth-rate and the death-rate of the blood- 
cells must be in close relation to each other in health; and 
some of the gravest disturbances arise from decided changes 
in the normal proportions of the cells {ancEmia, leucocytJie- 

B(jth the red and white corpuscles show, like all other cells 
of the organism, alterations corresponding to changes in the 
surrounding conditions. The blood may be withdrawn and its 
cells more readily observed than those of most tissues ; so that 
the study of the influence of temperature, feeding of the leuco- 
cytes, and the action of reagents in both classes of cells is both 
of practical importance and theoretic interest, and will well re- 
pay the student for the outlay in time and labor, if attention is 
directed chiefly to the results and the lessons they convey, and 
not, as too commonly happens, principally to the methods of 

The Chemical CompoBitlon of the Blood. — Blood has a decided 
but faint alkaline reaction, owing chiefly to the presence of 
sodium biphosphate (NajHPOi), a saline taste, and a faint odor 
characteristic of the animal group to which it belongs, owing 
probably to volatile fatty acids. The specific gravity of blood 
varies between 1045 and 1075, with a mean of 1055 ; the spe- 
cific gravity of the corpuscles being about 1105 and of the 
plasma 1037. This difference explains the sinking of the cor- 
puscles in blood withdrawn from the vessels and kept quiet. 
Much the same difliculties are encountered in attempts at the 
exact determination of the chemical composition of the blood, 
as in the case of other living tissues. Plasma alters its phys- 


ical and its chemical composition, to ■what extent is not exactly- 
known, when removed from the body. 

Composition of Seruin. — The fluid remaining after coagulation 
of the blood can, of course, be examined chemically with con- 
siderable thoroughness and confidence. 

By far the greater part of serum consists of water; thus, it 
has been estimated that of 100 parts the following statement 
will represent fairly, well the proportional composition : 

Water 90 parts ; 

Proteids 8 to 9 " 

Salines, fats, and extractives (small in 
quantity and not readily obtained 
free) 1 to 3 parts. 

The proteids are made up of two substances which can be 
distinguished by solubility, temperature at which coagulation 
occurs, etc., known as paraglobulin and serum-albumen, and 
which may exist in equal amount. 

It is not possible, of course, to say whether these substances 
exist as such in the living blood-plasma or not. 

The fats are very variable in quantity in serum, depend- 
ing on a corresponding variability in the plasma, in which 
they would be naturally found in greatest abundance after 
a meal. They exist as neutral stearin, palmitin, olein, and as 

The principal extractives found are urea, creatin, and allied 
bodies, sugar, and lactic acid. Serum in most animals contains 
more of sodium salts than the corpuscles, while the latter in 
man and some other mammals contain a preponderating quan- 
tity of potassium compounds. 

The principal salts of serum are sodium chloride, sodium bi- 
carbonate, sodium sulphate, and phosphate in smaller quantity, 
as also of calcium and magnesium phosphate, with rather more 
of potassium chloride. 

It is highly probable that this proportion also represents 
moderately well the composition of plasma, which is, of course, 
from a physiological point of view, the important matter. 

The Composition of the Corpuscles. — Taken together, the differ- 
ent forms of blood-cells make up from one third to nearly one 
half the weight of the blood, and of this the red corpuscles may 
be considered as constituting nearly the whole. 

The colorless cells are known to contain fats and glycogen, 
which, with salts, we may believe exist ip the living cells, and, 
in addition to the proteids, into which protoplasm resolves it- 


self upon the disorganization that constitutes its dying, lecithin, 
protagon, and other extractives. 

The prominent chemical fact connected with the red corpus- 
cles is their being composed in great part of a peculiar colored 
proteid compound containing iron. 

This will be fully considered later ; but, in the mean time, we 
may state that the hsemoglobin is itself infiltrated into the 
meshes or framework (stroma) of the corpuscle, which latter 
seems to be composed of a member of the globulin class, so well 
characterized by solubility in weak saline solutions. 

The following tabular statement represents the relative pro- 
portions in 100 parts of the dried organic matter of the red cor- 
puscles : 

Haemoglobin 90'54: 

Proteids 8'6r 

Lecithin 0"54: 

Cholesterin 0"25 


The quantity of salts is very small, less than one per cent 

So much for the results of our analyses ; but when we con- 
sider the part the blood plays in the economy of the body, it 
must appear that, since the life-work of every cell expresses it- 
self through this fluid, both as to what it removes and what it 
adds, the blood can not for any two successive moments be of 
precisely the same composition ; yet the departures from a nor- 
mal standard must be kept within very narrow limits, other- 
wise derangement or possibly death results. We think that, 
before we have concluded the study of the various organs of 
the body, it will appear to the student, as it does to the writer, 
that it is highly probable that there are great numbers of com- 
pounds in the blood, either of a character unknown as yet to 
our chemistry, or in such small quantity that they elude detec- 
tion by our methods ; and we may add that we believe the 
same holds for all the fluids of the body. The complexity of 
vital processes is great beyond our comprehension. 

It must be especially borne in mind that all the pabulum 
for every cell, however varied' its needs, can be derived from 
the blood alone ; or, as we shall show presently, strictly speak- 
ing from the lymph, a sort of middle-man between the blood 
and the tissues. 

The Quantity and the Distribution of the Blood. — Any attempt 


to estimate the total quantity of blood in the body of an animal 
by bleeding is highly fallacious for various reasons. It is im- 
possible to withdraw all the blood from the vessels by merely 
opening even the largest of them, and, if it were, the original 
quantity would be augmented by fluid absorbed into them dur- 
ing the very act. No method has as yet been devised that is 
free from objection, hence the conclusions arrived at as^to the 
total quantity of blood are not in accord ; and in the nature of 
the case no accurate estimate can be made, but about one thir- 
teenth to one fourteenth may be taken as a fair average ; so that 
in a man of one hundred and forty pounds weight there should 
be about ten pounds of blood ; but, of course, this will vary 
with every hour of the day and will be greatest after a meal. 

As an example of the methods referred to, we give Welck- 
er's, which is briefly as follows : The animal is bled to death 
from the carotid ; a sample of the defibrinated blood (1 cc.) is 
saturated with carbon monoxide (CO), which gives a perma- 
nent red color ; this diluted with 500 cc. of water furnishes a 
standard sample. The blood-vessels of the animal are washed 
out with a "6 per cent solution of common salt, but the out- 
flowing stream is colorless ; to this is added the fluid obtained 
by chopping up the tissues of the animal, steeping, washing 
out, and pressing. The whole is diluted to give the color of the 
standard solution, from which the amount of blood in this mixt- 
ure may be calculated, since every 500 cc. answers to 1 cc. of 
blood ; the blood obtained by bleeding can, of course, be accu- 
rately measured. 

It would be slightly more accurate to make the diluted 
blood of the animal operated upon the standard without treat- 
ment with carbon monoxi^. j-- 

"• Such a method, though the best yet devised, is open to ob- 
jection also, as will occur to most readers. ''•■ 

The relative quantities of blood in different parts of the 
body have been estimated to be as follows : 

Liver one fourth. 

Skeletal muscles " " 

Heart, lungs, large arteries, and veins . " " 
Other structures " " 

The significance of this distribution will appear later. 

The Coagulation of the Blood. — When blood is removed from 
its accustomed channels, it undergoes a marked chemical and 
physical change, termed clotting or coagulation. In the case 
of most vertebrates, almost as soon as the blood leaves the ves- 


sels it begins to thicken, and gradually acquires a consistence 
that may be compared to that of jelly, so that it can no longer 
be poured from the containing vessel. Though some have rec- 
ognized different stages as distinct, and named them, we think 
that an unprejudiced observer might fail to see that there 
were any well-marked appearances occurring invariably at the 
same ftioment, or with resting stages in the process, as with 
the development of ova. 

After coagulation has reduoed the blood to a condition in 
which it is no longer diffluent, minute drops of a thin fluid 
gradually show themselves, exuding from the main mass, 
faintly colored, but never red, if the vessel in which the clot 
has formed has been kept quiet so that the red corpuscles have 
not been disturbed ; and later it may be noticed that the main 
mass is beginning to sink in the center {cupping) ; and in the 
blood of certain animals, as the horse, which clots slowly, the 
upper part of the coagulum (crassamentum) appears of a 
lighter color, owing, as microscopic examination shows, to the 
relative fewness of red corpuscles. This is the buffy-coat, or, as 
it occurs in inflammatory conditions of the blood, was termed 
by older writers, the crusta phlogistica. It is to be distinguished 
from the lighter red of certain parts of a clot, often the result 
of greater exposure to the air and more complete oxidation in 
consequence. The white blood-cells, being lighter than the red, 
are also more abundant in the upper part of the clot {buffy- 
coat). If the coagulation of a drop of blood withdrawn from 
one's own finger be watched under the microscope, the red cor- 
puscles may be seen to run into heaps, like rows of coins lying 
against each other {rouleaux, Fig. 145), and threads of the 
greatest fineness are observed to radiate throughout the mass, 
gradually increasing in number, and, at last, including the 
whole in a meshwork which slowly contracts. It is the forma- 
tion of this fibrin which is the essential factor in clotting ; the 
inclusion of the blood-cells and the extrusion of the serum 
naturally resulting from its formation and contraction. 

The great mass of every clot consists, however, of corpus- 
cles ; the quantity of fibrin, though variable, not amounting to 
more usually than about "3 per cent in mammals. The forma- 
tion of the clot does not occupy more than a few minutes (two 
to seven) in most mammals, including man, but its contraction 
lasts a very considerable time, so that serum may continue to 
exude from the clot for hours. It is thus seen that, instead of 
the plasma and corpuscles of the blood as it exists within the 


living body, coagulation has resulted in the formation of two 
new products — serum and fibrin — differing both physically and 
chemically. These facts may be put in tabular form thus : 

Blood as it flows ( Liquor sangxdnis (plasma), 
in the vessels. (Corpuscles. 

Blood after co- j Coagulum | corpuscles, 
aguktion. j g^^^^ 

As fibrin may be seen to arise in the form of threads, under 
the microscope, in coagulating blood, and since no trace of it in 
any form has been detected in the plasma, and the process can 
.be accounted for otherwise, it seems unjustifiable to assume 
that fibrin exists preformed in the blood, or arises in any way 
prior to actual coagulation. 

Fibrin belongs to the class of bodies known as proteids, and 
can be distinguished from the other subdivisions of this group 
of substances by certain chemical as well as physical charac- 
teristics. It is insoluble in water and in solutions of sodium 
chloride; insoluble in hydrochloric acid, though it swells in 
this menstruum. 

It mq,y be whipped out from the freshly shed blood by a 
bundle of twigs, wires, or other similar arrangement present- 
ing a considerable extent of surface; and when washed free 
from red blood-cells presents itself as a white, stringy, tough 
substance, admirably adapted to retain anything entangled in 
its meshes. If fibrin does not exist in the plasma, or does not 
arise directly as such in the clot, it must have some antecedents 
already existing as its immediate factors in the plasma, either 
before or after it is shed. 

We shall here present certain facts, and examine the conclu- 
sions drawn from them afterward : 

1. Blood may be prevented from coagulating by receiving it 
in a solution of a neutral salt {magnesium sulphate, etc.), and 
upon certain chemical treatment precipitate a body which may 
be obtained by additional manipulation as a white, flaky sub- 
stance, that may be shown not to be fibrin, but which will 
clot and so give rise to this body. Such is the plasmine of 

2. By treatment of plasma with solid sodium chloride, two 
bodies with different coagulating points, but belonging to the 
same group of proteids {globulins, soluble in saline solutions), 
may be obtained, denominated paraglobulin and fibrinogen re- 


3. Paraglobulin may be obtained from serum also, and fibrin- 
ogen from certain fluids occurring normally {pericardial, pleu- 
ral, etc.) or abnormally (hydrocele fluid). 

4. Serum added to these fluids sometimes induces coagula- 

5. Coagulation may occur spontaneously in the above-men- 
tioned fluids when removed from the natural seat of their for- 

6. A preparation, made by extracting serum or the whipped 
(defibrinated) blood added to specimens of certain fluids when 
they do not coagulate spontaneously, as hydrocele fluid, often 
induces speedy clotting. 

7. This extract [fibrin-ferment) loses its properties on boil- 
ing, and a very small quantity suffices in most cases to induce 
the result. For these and other reasons this agent has been 
classed among bodies known as unorganised ferments, which 
are distinguished by the following properties : 

They exert their influence only under well-defined circum- 
stances, among which is a certain narrow range of tempera- 
ture, about blood-heat, being most favorable for their action. 

, They do not seem to enter themselves into the resulting prod- 
uct, but act from without as it were (catalytic action), hence a 
very small quantity suffices to effect the result. In all cases 
they are destroyed by boiling, though they bear exposure for 
a limited period to a freezing temperature. 

The conclusions drawn from the above statements are these : 
1. Coagulation results from the action of a fibrin -ferment on 
fibrinogen and paraglobulin. 2. Coagulation results from the 
action of a fibrin-ferment on fibrinogen alone. 3. Denis plasmine 
is made up of fibrinogen and paraglobulin. 

From observations, microscopic and other, it has been con- 

, eluded that the corpuscles play an important part in coagula- 
tion by furnishing the fibrin-ferment ; but the greatest diver- 
sity of opinion prevails as to which one of the morphological 
elements of the blood furnishes the ferment, for each one of 
them has been advocated as the exclusive source of this fer- 
ment by different observers. 

The above conclusions do not seem to us to follow neces- 
sarily from the premises. It might be true that a solution of 
fibrinogen, on having fibrin-ferment added to it, would clot, and 
yet it would not follow that such was the process of coagula- 
tion in the blood itself. All specimens of hydrocele fluid, and 
similar ones not spontaneously coagulable, do not clot when 


fibrin-ferment is added. Moreover, fibrin-ferment has not been 
isolated as an absolutely distinct chemical individual, free from 
all impurities. 

Because fibrinogen and paraglobulin give rise, under certain 
circumstances (it is asserted), to fibrin, and since plasmine acts 
likewise, it does not follow that plasmine contains these bodies. 
Further, it is stated that in the blood of crustaceans the clot 
arises from the' corpuscles chiefly, which run together and 
blend into a homogeneous mass. The fibrin so called in such 
a case differs not a little chemically, it could probably be shown, 
if our tests were delicate enough to discover it, from that which 
is denominated fibrin in other cases. " Fibrin-ferment " seems 
to have been used to cover much ignorance and unnecessary 
invention, as we shall endeavor to show later on ; and we can 
not but regard the reasoning in regard to the coagulation of 
the blood as evidence of an erroneous interpretation of certain 
facts on the one hand, and a large oversight of additional facts 
on the other hand. 

In the mean time we turn to certain well-known phenomena 
which bear a clear interpretation : 1. The blood remains fiuid 
in the vessels for some time after the death of an animal ; clots 
first in the larger vessels, and keeps fluid longest in the smaller 
veins. 2. The blood in the heart of a cold-blooded animal, as 
that of the frog or turtle, which will beat for days after the 
animal itself is dead, maintains its fluidity, but clots at once on 
removal. 3. The blood inclosed in a large vein removed be- 
tween ligatures does not coagulate for many hours (twenty- 
four to forty-eight). 

There are also facts of an opposite nature, thus : 1. When 
blood passes from a blood-vessel into one of the cavities of the 
body, it clots as if shed externally. 3. If a ligature be passed 
tightly around an artery so as to rupture the elastic coat, co- 
agulation ensues at the site of the ligature. 3. A similar 
clotting results when the inner coat of a blood-vessel is dis- 
eased, as in the case of roughening of the valves of the heart 
from inflammation, or the changes that give rise to aneurism 
of an artery. 4. A wire, thread, or other like foreign body, 
introduced into a vein, is speedily covered with fibrin. 

These facts, and others of like character, have been inter- 
preted as indicating that the living tissues of the blood-vessel or 
heart in some way prevent coagulation, but as to details there 
is difference of opinion. Some believe that the fibrin-ferment 
(essential to coagulation, according to their view) is formed by 


the corpuscles constantly, but in the above cases and during life 
is not effective because at once removed by the vessel walls ; 
while others are of opinion that the living cells composing these 
walls prevent the formation of the ferment. 

Even when injected into the blood-vessels, fibrin-ferment 
does not induce coagulation, nor does the constant death of the 
blood-cells, supposed thus to give rise to this substance, cause 

But the truth is, there is no necessity for all these somewhat 
artificial views, which seem to us to smack more of the labora- 
tory than of nature. 

We would explain the whole matter somewhat thus : What 
the blood is in chemical composition and other properties from 
moment to moment is the result of the complicated interaction 
of all the various cells and tissues of the body. Any one of 
these, departing from its normal behavior, at once affects the 
blood ; but health implies a constant effort toward a certain 
equilibrium, never actually reached but always being striven 
after by the whole organism. The blood can no more maintain 
its vital equilibrium, or exist as a living tissue out of its usual 
environment, than any other tissue. But the exact circum- 
stances under which it may become disorganized, or die, are 
legion ; hence, it is not likely that the blood always clots in 
the same way in all groups of animals, or even in the same 
group. The normal disorganization or death of the tissue re- 
sults in clotting ; but there may be death without clotting, as 
when the blood is frozen, in various diseases, etc. 

To say that fibrin is formed during coagulation expresses in 
a crude way a certain fact, or rather the resultant of many 
facts. To explain: When gunpowder and certain other ex- 
plosives are decomposed, the result is the production of cer- 
tain gases. If we knew these gases and their mode of com- 
position but in the vaguest way, we should be in much the 
same position as we are in regard to the coagulation of the 

There is no difficulty in understanding why the blood does 
not clot in the vessels after death so long as they live, nor why 
it does coagulate upon foreign bodies introduced into the blood- 
stream. So long as it exists under the very conditions under 
which it bCjgan its being, there is no reason why the blood 
should become disorganized (clot). It would be marvelous if 
it did clot, for then we could not understand how it could ever 
have been developed as a tissue at all. It is just as reasonable 


to ask why does not a muscle-cell become rigid, (clot) in the 
body during life. 

Probably in no field in physiology has so much work been 
done with so little profit as in the one we are now discussing ; 
and, as we venture to think, owing to a misconception of the real 
nature of the problem. We can understand the practical im- 
portance of determining what circumstances favor coagulation 
or retard it, both within the vessels and without them ; but 
from a theoretical point of view the subject has been exalted out 
of all proportion to its importance ; and we should not have 
dwelt so long upon it, or burdened the student with some of 
the theories we have stated, except in deference to the views 
held by so many physiologists. 

It is not surprising that, looking at the subject with a dis- 
torted mental perspective, one theory should have replaced an- 
other with such rapidity. It is, however, of practical impor- 
tance to the medical student to remember some of the factors 
that hasten or retard, as the case may be, the coagulation of the 
blood. Coagulation is favored by gentle movement, contact 
with foreign bodies, a temperature of about 38° to 40° C, addi- 
tion of a small quantity of water, free access of oxygen, etc. 
The process is retarded by a low temperature, addition of 
abundance of neutral salts, extract of the mouth of the leech, 
peptone, much water, alkalies, and many other substances. 
The excess of carbonic anhydride and diminution of oxygen, 
seem to be the cause of the slower coagulation of venous blood, 
hence the blood long remains fluid in animals asphyxiated. A 
little reflection suffices to explain the action of most of the fac- 
tors enumerated. Any cause which hastens the disintegration 
of the blood-cells must accelerate coagulation ; chemical changes 
underlie the changes in this as in all other cases of vital action. 
Slowing of the blood-stream to any appreciable extent likewise 
favors clotting, hence the explanation of the success of the 
treatment of aneurisms by pressure. It is plain that in all 
such cases the normal relations between the blood and the tis- 
sues are disturbed, and, when this reaches a certain point, death 
(coagulation) ensues, as with any other tissue. 

Clinical and Pathological. ^ — The changes in the blood that 
characterize certain abnormal states are highly instructive. If 
blood from an animal be injected into the veins of one of an- 
other species, the death of the latter often results, owing to non- 
adaptation to the blood already in the vessels, and to the tissues 
of the creature generally. The corpuscles break up — the change 



of conditions has been too great. Deficiency in the quantity of 
the hlood as a whole {oligcemia) causes serious change in the 
functions of the body ; but that a haemorrhage of considerable 
extent can be so quickly recovered from by many persons, 
speaks much for the recuperative power of the blood-forming 
tissues. Various kinds of disturbances in these blood-forming 
organs result in either deficiency or excess of the blood-cells, 
and in some cases the appearance of unusual forms of corpuscles. 
AncB,mia may arise from a deficiency either in the numbers 
or the quality of the red cells ; they may be too few, deficient 

FiQ. 157. 

Fig. 153.— Outlines of red corpuscles in a case of profound anaamia. 1, 1, normal corpuscles ; 
2, large red corpuscle— megalocyte ; 3, 3, very irregular forms— poikilocytes ; 4, very 
small, deep-red corpuscles— microcytes. 

Fig. 154. — Origin of microcytes from red corpuscles by process of budding and fission. Speci- 
men from red marrow. 

Fig. 155. — Nucleated red blood-corpuscles from blood in case of leukaemia. 

Fig. 156. — Corpuscles containing red blood-corpuscles. 1, from blood of child at term ; 3, from 
blood of a leukgemic patient. 

Fig. 157. — tt, 1, 2, 3, spleen-cells containing red blood-corpuscles. 5, from marrow ; 1, cell con- 
taining nine red corpuscles : 3, cell with reddish granular pigment ; 3, fusiform cell con- 
taining a single red corpuscle, c, connective-tissue corpuscle from subcutaneous tissue of 
young rat, showing the intracellular development of red blood-corpuscles. (Figs. 153-157, 
after Osier.) 

in size, or lacking in the normal quantity of haemoglobin. In 
one form (pernicious ancemia), which often proves fatal, a 
variety of forms of the red blood-cells may appear in the blood- 
stream ; some may be very small, some larger than usual, others 


nucleated, etc. Again, the white cells may be so multiplied that 
the blood may bear in extreme cases a resemblance to milk. 

In these cases there has been found associated an unusual 
condition of the bone-marrow, the lymphatic glands, the spleen, 
and, some have thought, of other parts. 

The excessive action of these organs results in the production 
and discharge into the blood-current of cells that are immature 
and embryonic in character. This seems to us an example of 
a reversion to an earlier condition. It is instructive also in that 
the facts point to a possible seat of origin of the cells in the 
adult, and, taken in connection with other facts, we may say, to 
their normal source. These blood-producing organs, having 
too much to do in disease, do their work badly — it is incom- 

Although the evidence, from experiment, to show that the 
nervous system in mammals, and especially in man, has an in- 
fluence over the formation and fate of the blood generally, is 
scanty, there can be little doubt that such is the case, when we 
take into account instances that frequently fall under the notice 
of physicians. Certain forms of anaemia have followed so di- 
rectly upon emotional shocks, excessive mental work and worry, 
as to leave no uncertainty of a connection between these and the 
changes in the blood ; and the former must, of course, have acted 
chiefly if not solely through the nervous system. 

It will thus be apparent that the facts of disease are in har- 
mony with the views we have been enforcing in regard to the 
blood, which we may now briefly recapitulate. 

Summary. — Blood may be regarded as a tissue, with a fluid 
matrix, in which float cell-contents. Like other tissues, it has 
its phases of development, including origin, maturity, and 
death. The colorless cells of the blood may be considered as 
original undifferentiated embryo cells, which retain their primi- 
tive character ; the non-nucleated red cells of the adult are the 
mature form of nucleated cells that in the first instance are 
colorless, and arise from a variety of tissues, and which in 
certain diseases do not mature, but remain, as they originally 
were at first, nucleated. When the red cells are no longer 
fitted to discharge their functions, they are in some instances 
taken up by amoeboid organisms (cells) of the spleen, liver, 

The chief function of the red corpuscles is to convey oxy- 
gen ; of the white, to develop as required into some more differ- 
entiated forni of tissue, act as porters of food-material, and 


probably to take up tbe work of many other kinds of cells 
when the needs of the economy demand it. The fluid matrix 
or plasma furnishes the lymph by which the tissues are direct- 
ly nourished, and serves as a means of transport for the cells 
of the blood. 

The chemical composition of the blood is highly complex, in 
accordance with the function it discharges as the reservoir 
whence the varied needs of the tissues are supplied ; and the 
immediate receptacle (together with the lymph) of the entire 
waste of the body ; but the greater number of substances exist 
in very minute quantities. The blood must be maintained of 
a certain composition, varying only within narrow limits, in 
order that neither the other tissues nor itself may suffer. 

The normal disorganization of the blood results in coagula- 
tion, by which a substance, proteid in nature, known as fibrin, 
is formed, the antecedents of which are probably very variable 
throughout the animal kingdom, and are likely so even in the 
same group of animals, under different circumstances ; and a 
substa.nce abounding in proteids (as does also plasma), known 
as serum, squeezed from the clot by the contracting fibrin. It 
represents the altered plasma. 

Certain well-known inorganic salts enter into the composi- 
tion of the blood — ^both plasma and corpuscles — but the princi- 
pal constituent of the red corpuscles is a pigmented, ferrugi- 
nous proteid capable of crystallization, termed haemoglobin. It 
is respiratory in function. 


That contractility, which is a fundamental property in some 
degree of all protoplasm, becoming pronounced and definite, 
giving rise to movements the character of which can be pre- 
dicted with certainty once the form of the tissue is known, finds 
its highest manifestation in muscular tissue. 

Very briefly, this tissue is made up of cells which may be 
either elongated, fusiform, nucleated, finely striated lengthwise, 
but non-striped transversely, united by a homogeneous cement 
substance, the whole constituting non-striped or involuntary 
muscle ; or, long nucleated fibers transversely striped, covered 
with an elastic sheath of extreme thinness, bound together 
into small bundles by a delicate connective tissue, these again 
into larger ones, till what is commonly known as a " muscle " 



is formed. This, in the higher vertebrates, ends in tough, 
inelastic extremities suitable for attachment to the levers it 
may be required to move (bones). 

Pio. 158. 

Fio. 159. 

Fig. 158. — Muscular fibers from tlie urinary bladder of the human subject. 1 x 200. (Sappey.) 
1, 1, 1, nuclei ; 3, 2, 3, borders of some of the fibers ; 3, 3, isolated fibers ; 4, 4, two fibers 
Joined together at 5. 

Fig. 159,— Muscular fibers from the aorta of the calf. 1 x 200. (Sappey.) 1, 1, fibers joined 
with each other ; 2, 2, 2, isolated fibers. 

Comparative. — The lowest animal forms possess the power of 
movement, which, as we have seen in Amoeba, is a result rather 
of a groping after food ; and takes place in a direction it is im- 
possible to predict, though no doubt regulated by laws definite 
enough, if our knowledge were equal to the task of defining 

Those ciliary movements among the infusorians, connected 
with locomotion and the capture of food, are examples of a pro- 
toplasmic rhythm of wonderful 
beauty and simplicity. ,,^ ..-•:..••' 

Muscular tissue proper first 
appears in the Cadenterata, but 
not as a wholly independent 
tissue in all cases. In many 
coelenterates cells exist, the low- 
er part of which alone forms a 
delicate muscular fiber, while 

the superficial portion {myoblast), composing the body of the 
cell, may be ciliated and is not contractile in any special 

Fig. 160.— Myoblasts of a jelly-fish, the Me- 
dusa Aurelia (Glaus). 


sense. The non-striped muscle-cells are most abundant among 
the invertebrates, though found in the viscera and a few other 
parts of vertebrates. This form is plainly the simpler and 
more primitive. The voluntary muscles are of the striped 
variety in articulates and some other invertebrate groups and 
in all vertebrates ; and there seems to be some relation between 
the size of the muscle-fiber and the functional power of the 
tissue — the finer they are and the better supplied with blood, 
two constant relations, the greater the contractility. 

Whether a single smooth muscle-cell, a striped fiber (cell), or 
a collection of the latter {muscle) be observed, the invariable 
result of contraction is a change of shape which is perfectly 
definite, the long diameter of the cell or muscle becoming 
shorter, and the short diameter longer. 

Ciliary Movements. — This subject has been already considered 
briefly in connection with some of the lower forms of life pre- 
sented for study. 

It is to be noted that there is a gradual replacement of this 
form of action by that of muscle as we. ascend the animal 
scale ; it is, however, retained even in the highest animals in 
the discharge of functions analogous to those it fulfills in the 

Thus, in Vorticella, we saw that the ciliary movements of 
the peristome caused currents that carried in all sorts of parti- 
cles, including food. In a creature so high in the scale as the 
frog we find the alimentary tract ciliated ; and in man himself 
a portion of the respiratory tract is provided with ciliated cells 
concerned with assisting gaseous interchange, a matter of the 
highest importance to the well-being of the mammal. As be- 
fore indicated, ciliated cells are found in the female generative 
organs, where they play a part already explained. 

It is a matter of ho little significance from an evolutionary 
point of view, that ciliated cells are more widely distributed in 
the foetus than in the adult human subject. 

As would be expected, the movements of cilia are affected 
by a variety of circumstances and reagents : thus, they are quick- 
ened by bile, acids, alkalies, alcohol, elevation of temperature 
up to about 40° C, etc. ; retarded by cold, carbonic anhydride, 
ether, chloroform, etc. 

In some cases their action may be arrested and re-estab- 
lished by treatment with reagents, or it may recommence with- 
out such assistance. All this seems to point to ciliary action as 
falling under the laws governing the movements of protoplasm 



in general. It is important to beai* in mind that ciliary action 
may go on in the cells of a tissue completely isolated from the 
animal to which it belongs, and though influenced, as just ex- 
plained, by the surroundings, that the movement is essentially 
automatic, that is, independent of any special stimulus, in which 
respect it differs a good deal from voluntary muscle, which 
usually, if not always, contracts only when stimulated. 

The lines along which the evolution of the contractile tissues 
has proceeded from the indefinite outflowings and withdraw- 
als of the substance of Amoeba up to the highly specialized 
movements of a striped muscle-cell are not all clearly marked 
out ; but even the few facts mentioned above suffice to show 
gradation, intermediate forms. A similar law is involved in 
the muscular contractility manifested by cells with other func- 
tions. The automatic (self -originated, independent largely of 
a stimulus) rhythm suggestive of ciliary movement, more 
manifest in the earlier developed smooth muscle than in the 
voluntary striped muscle of higher vertebrates, indicating 
further by the regularity with which certain organs act in 
which this smooth muscular tissue is predominant, a relation- 
ship to ciliary movement 
something in common as to 
origin — in a word, an evo- 
lution. And if this be 
borne in mind, we believe 
many facts will appear in 
a new light, and be invested 
with a breadth of meaning 
they would not otherwise 

The Irritability of Muscle 
and Nerve. — An animal, as 
a frog, deprived of its 
brain, will remain motion- 
less till its tissues have 
died, unless the animal be 
in some way stimula,ted. If 
a muscle be isolated from 
the body with the nerve to 
which it belongs, it will 
also remain passive ; but, 
if an electric current be passed into it, if it be pricked, pinched, 
touched with a hot body or with certain chemical reagents. 

FiQ. 161.— Nodes of Eanvier and lines of Fromann 
(Ranvier). A, Intercostal nerve of the mouse, 
treated with silver nitrate. B. Nerve-fiber from 
the sciatic nerve of a full-grown rabbit. A^ node 
of Ranvier ; JIf, medullary substance rendered 
transparent by the action of glycerine ; CY, axis- 
cylinder presenting the lines of Fromann, which 
are very distinct near the node. The lines are 
less marked at a distance from the node. 




contraction ensues ; the same happening if the nerve be thus 
treated instead of the muscle. The changes in the muscle and 
the nerve will be seen later to have much in common ; the mus- 
cle alone^ however, conirads, undergoes a visible change of form. 
Now, the agent causing this is a stimulus, and, as we have 
seen, may be mechanical, chemical, thermal, electrical, or nerv- 
ous. As both nerve and 
muscle are capable of 
being functionally af- 
fected by a stimulus, 
they are said to be irrita- 
ble; and, since muscle 
does not contract with- 
out a stimulus, it is said 
to be non-automatic. 

Now, since muscle is 
supplied with nerves as 
well as blood - vessels, 
which end in a peculiar 
way beneath the muscle- 
covering (sarcolemma) 
in the very substance of 
the protoplasm (end- 
plates), it might be that 
when muscle seemed to 
be stimulated, as above 
indicated, the responsive 
contraction was really 
due to the excited nerve 
terminals ; and thus has 
arisen the question. Is muscle of itself really irritable ? 

What has been said as to the origin of muscular tissue 

Fig. 162.— Mode of termination of the motor nerves 
(Flint, after Eouget). A. Primitive fasciculus of the 
thyro-hyold muscle of the human subject, and its 
nerve-tube : 1, 1, primitive muscular fasciculus ; 8, 
nerve-tube ; 3, medullary substance of the tube, 
which is seen extending to the terminal plate, where 
it disappears ; 4, terminal plate situated beneath the 
sarcolemma— that is to say, between it and the ele- 
mentary fibrillae ; B, 6, sarcolemma. B. Primitive 
fasciculus of the intercostal mu&cle of the lizard, in 
which a nerve-tube terminates : 1, 1, sheath of the 
nerve-tube ; 3, nucleus of the sheath ; 3, 3, sarco- 
lemma becoming continuous with the sheath ; 4, 
medullary substance of the nerve-tube, ceasing 
Abruptly at the site of the terminal plate ; 5, 5, ter- 
minal plate ; 6, 6, nuclei of the plate ; 7, 7, granular 
substance which forms the principal element of the 
terminal plate and which is continuous with the 
axis-cylinder ; 8, 8, undulations of the sarcolemma 
reproducing those of the iibrilles ; 9, 9, nuclei of the 

Fio. 163.— Intraflbrillar terminations of the motor nerve in striated muscle, stained with gold 

chloride (Landois). 

points very strongly to an affirmative answer, though it does 
not follow that a property once possessed in the lower forms of 


a tissue may not be lost in the higher ; hence the resort to ex- 
periments which have long been thought to settle the matter : 

1. The curare experiiherit may be thus performed : Lift up 
the sciatic nerve of a frog, and ligature the whole limb (ex- 
clusive of the nerve) so that no blood may reach the muscles ; 
then inject curare, which paralyzes nerves but not muscles, 
into the general circulation through the posterior lymph-sac. 
On stimulating the sciatic nerve the muscles of the leg beneath 
the ligature contract, while no contraction of the muscles of 
the opposite leg follows from stimulation of its sciatic nerve. 
In the lafter case the curare has reached the nerve terminals 
through the blood ; in the former, these were left uninfluenced 
by the poison. If, now, the muscle itself be directly stimulated 
in the latter case, contraction follows, from which it is con- 
cluded that curare has destroyed the functional capacity of the 
nerve (terminals), but not of the muscle. 

3. Stimulation of those parts of muscles in which no nervous 
terminations have been found, as the lower part of the sartorius 
muscle in the frog, is followed by contraction. 

3. Certain substances (as ammonia), when applied directly 
to the muscle, cause contraction, but are not capable of pro- 
ducing this effect when applied to the nerve. 

From these and various other facts it may be concluded that 
muscle possesses independent irritability. 


It is impossible to study the physiology of muscle to the 
best advantage without the employment of the graphic method ; 
and, on the other hand, no tissue is so well adapted for investi- 
gation by the isolated method — i. e., apart from the animal to 
which it actually belongs — as muscle ; hence the convenience of 
introducing at an early period our study of the physiology of 
contractile tissue and illustrations of the graphic method, the 
general principles of which have already been considered. 

The descriptions in the text will be brief, and the student is 
recommended to examine the figures and accompanying ex- 
planations with some care. 

Chronographs, Revolving CyUnders, etc. — Fig. 164 represents one 
of the earliest forms of apparatus for the measurement of brief 
intervals of time, consisting of a simple mechanism for pro- 



ducing the movement of a cylinder, which may be covered with 
smoked paper, or otherwise prepared to receive impressions 

made upon it by a point and capa- 
ble of being raised or lowered, and 
its movements regulated. The 
cylinder is ruled vertically into 
a certain number of spaces, so 
that, if its rate of revolution is 
known and is constant (very im- 
portant), the length of time of 
any event recorded on the sen- 
sitive surface may be accurately 
known. This whole apparatus 
may be considered a chrono- 
graph in a rough form. 

But a tuning-fork is the most 
reliable form of chronograph, 
provided it can be kept in con- 
stant action so long as required ; 
and is provided with a recording 
apparatus that does not cause 
enough friction to interfere with 
its vibrations. 

Fig. 166 illustrates one ar- 
rangement that answers these 
conditions fairly well. 

The marker, or chronograph, 
in the more- limited sense, is 
kept in automatic action by the fork interrupting the current 
from a battery at a certain definite rate answering to its own 
proper note. 

Fig. 164.— Original chronometer, devised by 
Thomas Young, for measuring minute 
portions of time (after McKendrick). 
a, cylinder revolving on vertical axis ; 
6, weight acting as motive power ; c, d, 
small balls for regulating the velocity 
of the cylinder ; e, marker recording a 
line on cylinder. 

Fio. 165.— Myographic tracing, such as is obtained when the cylinder on which It is written 
does not revolve during the contraction of the muscle (after McKendrick). 

• Marey's chronograph, which is represented at h above, and 
in more detail below, in Fig. 1G7, consists of two electro-magnets 
armed with keepers, between which is the writer, which has a 



little mass of steel attached to it, the whole working in unison 
with the tuning-fork, so that an interruption of the current 

Fio. 166.— Marey's chronograph as applied to revolving cylinder (after McKendrick). a. gal- 
vanic element ; 6, wooden stand bearing tuning-fork (two hundred vibrations per second); 
c, electro-magnet between limbs of tuning-fork ; d, e, positions for tuning-forks of one hun- 
dred and fifty vibrations per second ; /, tuning-fork lyin^ loose, which may be applied to 
d \ p, revolving cylinder ; A, electric chronograph kept in vibration synchronous with the 
tuning-fork interrupter. The current working the electro-magnet from a is interrupted at 
i. Foucault's regulator is seen over the clock-work of the cylinder, a Uttle to the right 
of 3. 

implies a like change of position of the writing-style, which is 
always kept in contact with the recording surface. 

Fia. 167.— Side view of Marey's chronograph (after McKendrick). a, a, coils of wire ; 6, &, 
keepers of electro-magnets ; c, vibrating style fixed to the steel plate e ; d, binding screws 
for attachment of wires ; ■*- from interrupting tuning-fork ; — to the battery. 

Fig. 177 shows the arrangements for recording a single 
muscle contraction, and Fig. 178 the character of the tracing 

A muscle-nerve preparation, which usually consists of the 
gastrocnemius of the frog with the sciatic nerve attached. 



clamped by a portion of the femur cut off with the muscle, is 
made, on stimulation, to raise a weighted lever which is at- 
tached to a point writing on a 
cylinder moved by some sort of 
clock-work. In this case the 
cylinder is kept stationary dur- 
ing the contraction of the mus- 
cle; hence the records appear 
as straight vertical lines, 
preparation, showing For recording movements of 

„ jle, sciatic nerve, and . ^ 

portion of femur of frog, for attachment great rapidltv. SO. that the in- 
to a mse (after Rosenthal). ^ ^ •' 

tervals between them may be 
apparent, such an apparatus as is figured below (Fig. 169) an- 
swers well, the vibrations of a tuning-fork being written on a 

FiQ. 168.— Muscle-nerve 
gastrocnemius muscl 

Fig. 169.— Spring myograph of Du Bois-Keymond (after Rosenthal). The arrangements for 
registering various details are similar to those for pendulum myograph (Fig. 177). 

blackened glass plate, shot before a chronograph by releasing 
a spring. 

Several records may be made successively by more compli- 
cated arrangements, as will be explained by another figure 

The Apparatus used for the Stimulation of Muscle. 

It is not only important that there should be accurate and 
delicate methods of recording muscular contractions, but that 



there be equally exact methods of applying, regulating, and 
measuring the stimulus that induces the contraction. 

Fig. 170 gives a representation of the inductorium of Du 
Bois-Reymond, by which either a single brief stimulation or a 

Fig. 170. — Du Bois-Reymond's inductorium (after Rosenthal), i, secondary coil ; c, primary 
coil ; b, electro-magnet ; ft, armature of hammer ; /, small movable screw. The current 
from battery, ascending metal pillar, passes alonehanuner, and by screw gets into primary 
coil, thus inducing current in secondary coil. By connection between primary coil and 
wires around soft iron of 6, iron becomes a magnet, hammer is attracted from screw /, 
and current thus broken ; but when this occurs, soft iron ceases to be a magnet neces- 
sarily, and, hammer springing back, the whole course of events is repeated. This may 
occur several hundred times in a second. The above may be clearer from diagram, Fig- 
171. By sliding secondary coil up and down, strength of induced current can be graduated. 

series of such repeated "with great regularity and frequency 
may be effected. The apparatus consists essentially of a pri- 

Fis. 171.— Diagrammatic representation of the working of Fig. 170 (after Rosenthal). 



mary coil, secondary coil, magnetic interrupter, and a scale to 
determine the relative strength of the current employed. The 
instrument is put into action by one or more of the various 
well-known galvanic cells, of which Daniell's are suitable for 
most experiments. 

Fis. 173. 

Fig. 178. 

Fig. 172.— PflUger's rayoffraph. The muscle may be fixed to the vise C in the moist-chamber, 
the vise connecting with the lever E E, the point of which touches the plate of smoked 
glass G. The lever is held in equipoise by H. When weights are placed in scale-pan F, 
the lever writes the degree of extension effected (after Rosenthal). 

Fig. 173.— Tetanizing key of Du Bois-Eeymond (after Rosenthal). Wires may be attached at b 
and c. When d is down the current is " shor(>circuited," i. e., does not pass through the 
wires, but direct from c through d to 6, or the reverse, since 6, c, d are of metal, and, on 
account of their greater cross-section, conduct so much more readily than the wires, a is 
an insulating plate of ebonite. This form of key is adapted tor attachment to a table, etc. 

The access to, or exclusion of the current from, the induc- 
torium is effected by some of the forms of keys, a specimen of 
which is illustrated in Fig. 173. 

The moist chamber, or some other means of preventing the 
drying of the preparation, which would soon result in impaired 
action, followed by death, is essential. A moist chamber con- 
sists essentially of an inclosed cavity, in which is placed some 
wet blotting-paper, etc., and is usually made with glass sides. 
The air in such a chamber must remain saturated with moist- 



A good knowledge of the subject of electricity is especially 
valuable to the student of physiology. But there are a few ele- 
mentary facts it is absolutely necessary to bear in mind : 1. An 
induced current exists only at the moment of making or break- 
ing a primary (battery) current. 2. At the moment of making, 
the induced current is in the opposite direction to that of the 
primary current, and the reverse at breaking. 3. The strength 
of the induced current varies with the strength of the primary 
current. 4. The more removed the secondary coil from the 
primary the weaker the current (induced) becomes. 

The clock-work mechanism and its associated parts, as seen 
in Fig. 174, on the right, is usually termed a myograph. 

Fig. 174. — Arrangement of apparatus for transmission of muscular movement by tambours 
(after McKendriclc). a, galvanic element ; 6, primary coil ; c, secondary coil of inducto- 
rium ; d, metronome for interrupting primary circuit when induction current is sent to 
electrodes k ; A, forceps for femur ; the muscle, which is not here represented, is attached 
to the receiving tambour gf, by which movement is transmitted to recording tambour e, 
which writes on cylinder /. 

Instead of muscular or other movements being communi- 
cated directly to levers, the contact may be through columns 

FiQ. 175.— Tambour of Marey (after McKendriok). a, metallic case ; 6, thin India-rubber mem- 
brane ; c, thin disk of aluminium supporting lever d, a small portion of which only is repre- 
sented ; e, screw for placing support of lever vertically over c ; /, metallic tube communi- 
cating mth cavity of tambour tor attachment to an India-rubber tube. 



of air, which, it will be apparent, must be capable of communi- 
cating very slight changes if the apparatus responds readily to 
the alterations in volume of the inclosed air. 

Fig. 175 represents a Marey's tambour, which consists essen- 
tially of a rigid metallic case provided with an elastic top, to 
which a lever is attached, the whole being brought into com- 
munication with a column of air in an elastic tube. The work- 
ing of such a mechanism will be evident from Figs. 174 and 176. 

Fig. 176. — Tambours of Marejr arranged -for transmission of movement (after McKendrick). 
a, receiving tambour ; 6, india-rubber tube ; c, registering tambom* ; d, spiral of wire, 
owing to elasticity of which, when tension is removed from a, the lever ascends. 

The greatest danger in the use of such apparatus is not fric- 
tion but oscillation, so that it is possible that the original move- 
ment may not be expressed alone or simply exaggerated, but 
also complicated by additions, for which the apparatus itself is 

Apparatus of this kind is not usually employed much for 
experiments with muscle; such an arrangement is, however, 
shown in Fig. 174, in which all will be seen — a metronome, the 
pendulum of which, by dipping into cups containing mercury, 
makes the circuit. Such or a simple clock may be utilized for 
indicating the longer intervals of time, as seconds. 

A Single Simple Muscular Contraction. 

Experimental Facts. — The phases in a single twitch or muscu- 
lar contraction may be studied by means of the pendulum 



Fig. 177.— Diagrammatic representation of the pendulum myograph. The smoked-glass plate, 
A, swing8 with a pendulum, B. Before an experiment is commenced the pendulum ia 
raised up to the right and kept in position by the tooth, a, catching on the springe-catch, b. 
On depressing the catch, 6, the glass plate being set free swings into the new position indi- 
cated by the dotted lines, and is held there by the tooth, a', meeting the catch, h'. In the 
course of its swing the tooth, a, coming into contact with the projecting steel rod, c, knocks 
it to one side, into the position indicated by the dotted line, c'. The rod, c, is in electric 
continuity with the wire, x, of the primary coil of an induction machine. In like manner 



the'screw, d, is in electric continuity with the wire, y, of the same primary coil. The screw, 
d, and the rod, c, are provided with platinum points, and both are insulated by means of 
ttie ebonite block, e. The circuit of the primary coil to which x and y belong is closed as 
'long as c and d are in contact. When in its swing the tooth, a', knocks c away from d, the 
■circuit is immediately broken, and a *' breaking " shock is sent through the electrodes con- 
nected with the secondary coil of the machine, and so through the nerve. The lever, Z, the 
end only of which is shown in the figure, is brought to bear on the glass plate, and when at 
rest describes an arc of a circle of large radius. The tuning-fork, / (ends only seen), serves 
to mark the time (after Foster). 

myograph. (Fig. 177). It consists of a heavy pendulum, which 
swings from a position on the right to a corresponding one on 
the left, where it is secured by a catch. During the swing of 
the pendulum, which carries a smoked glass plate (by means 
of arrangements more minutely described below the figure), a 
tuning-fork writes its vibrations on the plate, on which is 
inscribed the marking indicating the exact moment of the 
breaking of an electric current, which gives rise to a muscle 
contraction that is also recorded on the plate. 

The tracing on analysis presents : 1. The record of a tuning- 
fork making one hundred and eighty vibrations in a second. 
3. The pajallel marking of the lever attached to the muscle 
before it began to rise. 3. A curve, at first rising slowly, and 
then rapidly to a maximum. 4. A curve of descent similar in 
character, but somewhat more lengthened. 

We may interpret this record somewhat thus : 1. A rise of 
the lever answering to the shortening of the muscle to which it 

Fig. 178.— Muscle-curve obtained by the pendulum myograph (Poster). Bead from left to 
right. The latent period is indicated by the space between a and 6, the length of which ia 
measured by the waves of a tuning-fork, making one hundred and eighty double vibrations 
in a second ; and in Uke manner the duration of the other phases of the 'contraction may 
be estimated. 

is attached following upon the momentary induction shock, as 
the entrance of the current into the nerve, the stimulation of 
which causes the contraction, may be called. 2. A period before 
the contraction begins, which, as shown by the time marking, 

occupies in this case 3-^, or about -^oi a second. 

In the tracing 
the upward curve indicates that the contraction is at first rela- 


tively slow, then more rapid, and again slower, till a brief sta- 
tionary period is reached, when the muscle gradually but rap- 
idly returns to its previous condition, passing through the same 
phases as during contraction proper. In other words, there is 
a period of rising and of falling energy, or of contraction, and 
relaxation. 4. A period during which invisible changes, as 
will be explained later, are going on, answering to those in the 
nerve that cause the molecular commotion in muscle which 
precedes the visible contraction — the latent period, or the 
period of latent stimulation. 

The facts may be briefly stated as follows : The stimulation 
of a muscle either directly or through its nerve causes contrac- 
tion, followed by relaxation, both of which are preceded by a 
latent period, during which no visible but highly important 
molecular changes are taking place. The whole chain of events 
is of the briefest duration, and is termed a muscle contraction. 
The tracing shows that the latent period occupied rather more 
than xfir second, the period of contraction proper about y^, 
and of relaxation yf ^ second, so that the whole is usually begun 
and ended within -^ second; yet, as will be learned later, 
many chemical and electrical phenomena, the concomitants of 
vital change, are to be observed. 

In the case just considered it was assumed that the muscle 
was stimulated through its nerve. Precisely the same results 
would have followed had the muscle been caused to contract 
by the momentary application of a chemical, thermal, or me- 
chanical stimulus. 

If the length of nerve between the point of stimulation and 
the muscle was considerable, some difference would be observed 

Fig. 179.— Diagrammatic representation of the measurement of velocity of nervous impulse 
(Foster). Tracing taken by pendulum myograph (Fig. 177). The nerve of same muscle- 
nerve preparation is stimulated in one case as far as possible from muscle, in the other as 
near to it as possible. Latent period is a6. ab\ respectively. Difference between ab and 
ab' indicates, of course, length of time occupied by nervous impulse in traveling along 
nerve from distant to near point. 

in the latent period if in a second case the nerve were stimu- 
lated, say, close to the muscle. This is represented in Fig. 179, 



in whicli it is seen that tlie latent period in the latter case is 
shortened by the distance from V to b, which must be owing 
to the time required for those molecular changes which, occur- 
ring in a nerve, give rise to a contraction in the muscle to which 
it belongs ; in fact, we have in this method a means of estimat- 
ing the rate at which these changes pass along the nerve — in 
other words, we have a means of measuring the speed of the 
propagation of a nervous impulse. , The estimated rate is for the 
frog twenty-eight metres per second, and for man about thirty- 
three metres. As the latter has been estimated for the nerve, 
with its muscle in position in the living body, it must be re- 
garded rather as a close approximation than as exact as the 
other measurements referred to in this chapter. 

It will be borne in mind that the numbers given as repre- 
senting the relative duration of the events vary with the ani- 
mal, the kind of muscle, and a variety of conditions affecting 
.the same animal. 

Tetanic Contraction. 

It is well known that a weight may be held by the out- 
; stretched arm with apparently perfect steadiness for a few 
seconds, but that presently the arm begins to tremble or vi-. 
brate, and soon the weight must be dropped. Tte arm was 
maintained in its position by the joint contraction of several 
muscles, the action of which might be described (traced) . by a 
writer attached to the hand and recording on a moving sur- 
face. Such a record would indicate roughly what had hap- 
pened ; but the exact nature of a muscular contraction in such 
a case can best be learned by laying bare a single muscle, say 
in the thigh of a frog, and arranging the experiment so that a 
graphic record shall be made. 

Using the apparatus previously described (Fig. 177), a second 
induction shock may be sent into the muscle before the effect 

Fig. 180.— Tracing of a double muscular contraction (Foster). A second induction shoclc was 
sent into muscle when it had so far completed its contraction as is indicated by beginning 
of second rise. Dotted line indicates what the curve would have been but for this. 



of the first has passed away, the result depending on the phase 
of the contraction, during which the stimulus acts on the mus- 
cle. Thus, if a second shock be applied during the latent pe- 
riod, no visible change in the nature of the muscle-curve can be 
seen ; but if during one of the other phases of contraction, a re- 
sult like that figured below (Fig. 180) follows. If a series of 
such shocks be sent into the muscle before its contraction pe- 
riod is over, a succession of curves may be superposed on one 

■ '■'■' 

Fig. 18t. — Curve of imperfect tetanic contraction (Foster). Uppermost tracing indicates con- 
tractions of muscle ; intermediate, when the shocks were given ; lower, time-markings of 
intervals of one second. Curve to be read, like others, from left to right, and illustrate at 
the end a " contraction remainder." 

another, to the total height of which, however, there is a limit, 
no matter what the strength of the stimulus used. 

If the stimuli follow each other with a certain rapidity, such 
a tracing as that represented in Fig. 181 is obtained ; and if the 
rapidity of the stimulation exceeds a certain rate, the result is 
that seen in Fig. 182. 

Fia. 188.— Curve of complete tetanic contraction (Foster). 

It is possible to see in these tracings a genetic relation, the 
second figure being evidently derivable from the first, and the 
third from the second, by the fusion of all the curves into one 
straight line. 

If a muscle, isolated as we have described, be watGhed dur- 
ing the period that it is writing the second and the third 


tracing, it may be observed that, during that corresponding to 
the former, though it is shortened, it does not remain equally so 
throughout, while during the writing of the third tracing there 
is no variation in its condition appreciable by the eye. What 
has happened is this : The muscle during the condition figured 
in the second tracing has periods of alternating contraction and 
partial relaxation, but during the third case the latter phase 
has been apparently omitted — the muscle remains in continuous 
contraction. In reality this is not the case unless we are mis- 
taken as to the meaning of the muscle-sound. 

The Muscle Tone. — ;There are a number of experimental facts 
from which important conclusions have been drawn, to which 
attention is now directed : 

1. It has been found that a sound may be heard in a still 
room when one brings the muscles of mastication into action 
by biting hard; or listens over a contracting biceps with a 
stethoscope, etc. 

2. "When the wires of a telephone (communicator) are con- 
nected with a muscle, a sound is heard during the contraction 
of the muscle. 

From these facts it was concluded that a muscle when con- 
tracting gives rise to a sound ; that tetanus, as the form of con- 
traction we are describing is called, is essentially vibratory in 
character, which seems to answer to the graphic representations 
from a muscle when in tetanic contraction, and is in harmony 
with the case to which we called attention at the commence- 
ment of this subdivision of the subject. The note heard cor- 
responded, in the case of an isolated muscle, to the number of 
stimulations per second ; while for muscles made to contract by 
the will the note was always the same, answering to about 
forty vibrations per second; but as forty stimuli are not re- 
quired within this period of time to induce tetanus, it was 
thought that this note was probably the harmonic of a lower 
one answering to twenty vibrations in a second. 

It has been recently shown that a very much smaller num- 
ber of vibrations of the muscle can give rise to an audible 
sound, so that the explanation it would seem must now be 
modified; and it is likely that some peculiarities of the ear 
itself must be taken into the account in the explanation. In 
making the observations referred to above (in 1), the student 
will find it very important to be on his guard against sources 
of error, especially with the use of a stethoscope. 

We may safely conclude that, at all events, most of the mus- 


ciilar contractions occurring within the living body are tetanic 
— i. e., the muscle is in a condition of shortening, with only very 
brief and slight phases of relaxation ; and that a comparatively 
small number of individual contractions suffice for tetanus 
when caused by the action of the central nervous system; 
though, as proved by experiments on muscle removed from the 
body, they may be enormously increased. While a few stimu- 
lations per second suffice to cause tetanus, it will also persist 
though thousands be employed. 

The Strength of the Stimulus. — We have assumed that in the 
cases of contraction thus far considered the stimulus was ade- 
quate to produce the full amount of contraction, or as much as 
could be obtained. Such a contraction and such a stimulus are 
spoken of as maximal; but the stimulus might fall a little 
short of this, and is then termed sub-maximal ; or it may be re- 
garded from the point of view of being the least that will cause 
a contraction, and is then the minimal stimulus. 

It is important to note that any sudden change in an electric 
current will act as an excitant to muscular contraction, but 
that very considerable changes in the strength of the current if 
made gradually do not react on the muscle. It sometimes hap- 
pens that a sudden onward push of the secondary coil of an 
induction-machine will produce either a tetanus (though the 
terminal wires or electrodes were arranged for a single induc- 
tion shock) or what is known as a supermaximal contraction — 
i. e., one in excess of what could be obtained by more gradual 
advances, which have no effect usually after a certain maxi- 
mum of contraction is reached. This, we think, a matter of 
considerable practical importance, and shall refer to its signifi- 
cance in a later chapter. 

Since the opening or closing of a key which makes or breaks 
the current really implies a very great change in the strength 
of the current affected suddenly — that is in fact from to some 
+ quantity or the reverse — we find that usually the most marked 
contractions occur only at these times, and this holds, whether 
the current be slowly or rapidly made and broken (inter- 

The nerve being the natural means of conveying a stimu- 
lus, it is easy to understand how the contraction happens to 
follow most perfectly and with less strength of stimulus when 
this structure is excited. 


The Changes in a Muscle during Conteaction. 

Though the change in form is very great during the con- 
traction of a muscle, the change in bulk is almost inappreci- 
able, amounting to a diminution of not more than about tsV? 
of the volume. In fact, according to the latest investigator, 
there is no diminution whatever. A series of levers may be 
laid on a muscle or the columns of air in a series of Marey's 
tambours may be influenced by the contracting muscle, and 
from some such apparatus a graphic record like that seen in 
Fig. 183 may be obtained. 

It is to be observed that the contraction passes along the 
muscle in the form of a wave, the size and speed of which are 

FiQ. 183. — Tracing of the propagation o£ the muscular wave. Chronographic tracing, one 
hundred vibrations per second underneath (Marey). 

susceptible of measurement. For the frog the wave-length is 
estimated at from 200 to 400 mm., and the velocity at about 3 
to 4 metres per second. 

It is probably rather greater in the muscles of mammals 
and greater under the more natural conditions of the muscle in 
the intact living body. 

But since the fibers of striped muscle are of very limited 
length (30 to 40 mm.), it would seem that a contraction origi- 
nating in one fiber must be capable of initiating a similar 
action in its neighbor ; and, as the ends of the fibers lie in con- 
tact, it is easy to understand how the wave of contraction 
spreads. Normally, the contraction must pass from about the 
center of the muscle-cell where the nerve terminates in the 

The microscopic changes occurring in contracting muscle 
are not well understood. The living muscle of a beetle's thigh 
when placed under a microscope may be seen in contraction — a 
sight of the most striking nature, reminding one of a billowy, 
tempestuous sea, and by the use of reagents the waves of con- 
traction may be fixed. 

It may be stated that the parts distinct before remain so 



during contrition, and that all parts of the muscle-substance 
seem to share in the changes of form involved. 

The Elasticity of Muscle. 

In proportion as bodies tend to resume their original form 
when altered by mechanical force are they elastic, and the ex- 
tent to which they do this marks the limit of 
their elasticity. 

If a muscle (best one with bundles of fibers 
of about equal length and parallel arrange- 
ment) be stretched by a weight attached to 
one end, it will, on removal of the extending 
force, return to its original length ; and if a 
series of weights which differ by a common 
increment be applied in succession and the 
degrees of extensions compared, as may be 
done by the graphic method, it will be appar- 
ent that the increase in the extension does not 
exactly correspond with the increment in the 
weight, but is proportionally less. With an 
inorganic body, as a watch-spring, this is not 
the case. 

Further, the recoil of the muscle after the 
removal of the weight is not perfect for all 
weights ; but within certain narrow limits 
this is the case, i. e., the elasticity of muscle, 
though slight (for it is easily over-extended), 
is perfect. When once a muscle is over-ex- 
tended, so weighted that it can not reach its 
original length almost at once, it is very slow 
to recover, which explains the well-known 
duration of the effects of sprains, no doubt 
owing to some profound molecular change fig is4-du Bois-Eey 
associated with the stretching. SeiiuSfoPS^uc 

The tracings below show at a glance the f^f^f°°iSsenttfat 
difference between the elasticity of muscle attachlTd to musciet 
and of ordinary bodies. ^°,e^. ""^^"^"i ^''^'^ 

It is a curious fact that a muscle during 
the act of contraction is more extensible than when passive ; a 
disadvantage from a purely physical point of view, but proba- 
bly a real advantage as tending to obviate sprain by prevent- 
ing too sudden an application of the extending force. 


It will be borne in mind that the limbs are held together as 
by elastic bands slightly on the stretch, owing to the elasticity 

Fio. 185.— Illustrations of the difference in elasticity of inanimate and living matter (after 
Yeo). 1. Shows graphically behavior of a steel spring under equal increments of weight. 
3. A similar tracing obtained from an India-rubber band. 3. The same from a frog's 
muscle. Note that the extension decreases with egual increments of weight, and that the 
muscle fails to return to the original position (abscissa) after removal of the weight. 

of the muscles. Now, as seen in many tracings of muscular 
contraction, there is a tendency to imperfect relaxation after 
contraction — the contraction remainder or elastic after-effect, 
which can be overcome by gentle traction. In the living body, 
the weight of the limbs and the action of the stretched muscles 
on the side of the limb opposite to that on which the muscles 
in actual contraction are situated, combine to make the action 
of the muscle more perfect by overcoming this tendency to im- 
perfect relaxation, which is probably less marked, independent 
of these considerations, in the living body. This elasticity of 
living muscles, which is completely lost on death, is a fair 
measure of their state of health or organic perfection. Hence 
that hard (elastic recoil) feeling of the muscles in young and 
vigorous persons, especially athletes, in whom muscle is brought 
to the highest degree of perfection. 

This property is then essentially the outcome of vitality, 
which is in a word the foundation of the difiEerences noted be- 
tween the elasticity of inorganic and organic bodies. A mus- 
cle, the nutrition of which is suffering from whatever cause, 
whether deficient blood-supply, fatigue, or actual disease, is 
deficient in elasticity. We wish to emphasize these relations, 
for we consider it very important to avoid regarding vital phe- 
nomena in the light of physics merely, which the employment 
of the graphic method (and indeed all methods by which we re- 
move living things out of their normal relations) fosters. 

Electrical Phenomena of Muscle. — Certain pieces of apparatus 



not as yet referred to are required to demonstrate the electrical 
condition of muscle. The galvanometer suitable for physio- 
logical experiments is one having very many coils of extreme- 
ly fine wire, and so adapted to indicate the presence of currents 
of slight intensity. 

In order that it may be ascertained definitely that the cur- 
rents that deflect the galvanometer needle do not originate out- 
side of the muscle itself, non-polarizable electrodes very care- 
fully made must be used, for the contact of ordinary metallic 
electrodes with living tissues suffices of itself to generate an 
electric current, as may be simply illustrated to one's self by 
placing two coins, one silver and the other copper, in contact 
with the upper and under surfaces of the tongue respectively, 
and meeting in front ; a peculiar taste results from the current 

The construction of the non-polarizable electrodes common- 
ly employed, and as arranged for use, is diagrammatically rep- 
resented below (Fig. 186). 

Assuming the apparatus for the detection of electrical cur- 
rent in muscle to be in working order, a muscle from one of 

FiQ. 186.— Non-polarizable electrodes of Du Bois-Eeymond (after Rosenthal. At c, clay tip, 
moistened with saline solution, is laid on muscle. Glass cylinder a is filled with strong 
solution of zinc sulphate, a good conductor, by which current is conveyed to amalgamated 
zinc plate b, and thence to galvanometer. 

the cold-blooded animals, prepared as rapidly and carefully as 
possible, avoiding all contact with foreign bodies, is cut across 
the ends transversely, and placed on pads of bibulous paper 
moistened with physiological ('GO-^S per cent) saline solution. 
The non-polarizable electrodes connected with the galvanome- 
ter are brought in contact with the muscle. What results 
depends on the parts of the muscle that touch the electrodes, 
and is represented diagramatically in Fig. 187. 

It will be observed that the diagram indicates that between 
no current and the strongest obtainable there are all shades of 



strength, according to the parts of the muscle connected by the 
electrodes. The strongest is that resulting when the superfi- 

FiQ. 187.— Representation of electrical currents in a muscle-rhombus (after Rosentlial). 

cial equator and the transverse center are connected ; and it is 
found that the nearer these points are approached the stronger 
the current becomes, as is indicated by the greater extent of 
swing of the galvanometer needle. In connection with these sur- 
prising phenomena, one naturally inquires whether such a mus- 
cle-current, for such it must be, is natural or artificial. Does 
such exist in a living muscle in its position in the body, or has 
the injury done to a muscle in its preparation by section, re- 
moval from the usual conditions of nutrition, and such like 
changes, been the cause of the current ? 

After much investigation, by some of the ablest physiolo- 
gists of the day, different answers are returned to these queries. 

Du Bois-Reymond maintains that such currents are natural, 
and may be obtained from muscle contracting in situ ; while 
Hermann and others believe that such a current is owing to 
the injury done by the section, and that the current from the 
equator to the poles of the section is due to the fact that the 
injured part is negative to the uninjured region. 

It is a fact that if the current be led off from an exposed 
muscle prior to section, it is relatively very weak. Further, 
the electrodes placed on the uninjured ventricle of an animal's 



heart convey no current to the galvanometer ; but after section, 
as in the case of a skeletal muscle, the usual result follows. 
All observers, however, are agreed that a current is produced 
during contraction. Those not believing in that just referred 
to above (" current of rest "), term this one the " current of 
action " ; while the other school names it the negative variation 
of the current of rest, inasmuch as the galvanometer needle 
swings in the opposite direction indicating, as they say, a 
diminution in the original current. 

The presence of this undisputed current can be made evident 
by a simple experiment, without the use of any of the elabo- 
rate apparatus noticed above. Let two frog's limbs, with the 

Fig. 188.— Arrangement of parts to show 
secondary contraction in muscle 
(after Rosenthal). 

Fig. 189.— The same when the primaiy cause 
is in nerve (after Rosenthal). - 

nerves belonging to them, be prepared in good condition and 
arranged as in Fig. 188, so that the nerve of A rests along the 
thigh of B. On stimulating the nerve of B, the muscular effect 
in this limb is answered by a similar one in A. That this is not 
necessarily due to escape of the current upon the nerve of A, 
may be shown by putting a ligature around the nerve of B below 
the point of application of the current and moistening it so as 
to allow of the free passage of the current. In such case stimu- 
lation of the nerve of B gives wholly negative results, because 
the ligature has destroyed physiological (molecular) continuity, 
though it does not prevent the passage of the current. More- 


over, the result may be obtained by other than electrical 

The explanation of these phenomena of the "rheoscopic 
frog" (physiological rheoscope) is simply that the electrical 
condition of B has been suddenly changed by the passage of 
the current into the nerve, and that this difference of electrical 
condition (potential) between the muscle of B and A's nerve 
suffices to stimulate the muscle of A (one is in fact + and the 
other — ) ; hence the stimulus and the contraction, the nature 
of which in A is the same as that in B — i. e., a single twitch 
in B gives rise to the same in A, and a tetanic contraction to a 
tetanic contraction. Plainly the contraction of A must be due 
to a current in B, hence the proof that a current actually exists 
during the contraction of a muscle. It may be noted that a 
mere prick of B will arouse in it a contraction which is fol- 
lowed by the same result as before in A, so that in this we can 
exclude the original stimulating current altogether as a pos- 
sible source of fallacy, as stated above. But one of the most 
striking proofs that there is a current of action (or negative 
variation), is obtained by placing the nerve of such a prepara- 
tion as that represented in B on a contracting mammalian heart ; 
with each systole there is a spasm of the frog's leg. 

It is important to note that the electric current of muscle, 
however viewed, is an event of the latent period. It is asso- 
ciated with the chemical and all the other molecular changes 
of which the actual contraction is but the outward and visible 
sign ; and since the currents of rest have an appreciable dura- 
tion, wane with the vitality of the tissue, and wholly disappear 
at death, they must be associated with the fundamental facts 
of organic life ; for it is to be remembered that electrical cur- 
rents are not confined to muscle, but have been detected in the 
developing embryo, and even in vegetable protoplasm. Though 
the evidence is not yet complete, it seems likely that electrical 
phenomena may prove to be associated with (we designedly 
avoid any more definite expression) all vital phenomena. 

Chemical Changes in Muscle. — In an animal, at a variable 
period after death, the muscles become rigid, producing that 
stiffness {rigor mortis) so characteristic of a recent cadaver. 

The subject can be studied in some of its aspects to great 
advantage in an isolated individual muscle. 

Three changes in a muscle that has passed into death rigor 
are constant and pronounced. The living muscle, either alka- 
line or neutral in reaction, has become decidedly acid; an 


abundance of carbonic anhydride is suddenly given off ; and 
myosin, a specific proteid, has been formed. That these phe- 
nomena have some indissoluble connection with each other so 
far as the first two at least are concerned, while not absolutely 
certain, seems probable, as will be learned shortly. 

It will be borne in mind that muscle-fibers are tubes con- 
taining semifluid protoplasm, and that a coagulation of the 
latter must give rise to general rigor. This protoplasmic sub- 
stance can be extracted at a low temperature from the muscles 
of the frog, fend, as the temperature rises coagulates like blood, 
giving rise to a clot (myosin) and muscle-serum, a fluid not 
very unlike the serum of blood. 

This myosin can also be extracted from dead rigid mus- 
cles by ammonium choride, etc. It resembles the globulins 
gei^erally, but is less soluble in saline solutions than the globu- 
lin of blood (paraglobulin) ; is less tough than fibrin ; has a 
very low coagulating point (55° to 60° C.) ; and is somewhat 
jelly-like in appearance. The clotting of blood and of muscle 
is thus analogous, myosin answering to fibrin, and there being 
a serum in each case, both processes marking the permanent 
disorganization of the tissue. The reaction seems to be due to 
the formation of a kind of lactic acid, probably sarolactic; 
though whether due to excessive production of this acid, on 
the death of the muscle, which for some reason does not remain 
free in the living muscle, or whether sarcolactic acid arises as 
a new product, is uncertain. It is certain that the acid reaction 
of dead muscle is not owing to carbonic acid, for the reddened 
litmus does not change color on drying. 

That a muscle in action does use up oxygen and give off 
carbonic anhydride can be definitely proved ; though it is 
equally clear that the life, of a muscle is not dependent on a 
constant supply of oxygen as is that of the individual, for a 
muscle can live, even contract long and vigorously, in an atmos- 
phere free from this gas, as in nitrogen. 

From the suddenness of the increase of carbonic anhydride, 
the onset of death and rigor mortis has been compared to an 

After this the muscle becomes greatly changed physically : 
its elasticity and translucency are lost; there is absence of 
muscle-currents ; it is wholly unirritable, is less extensible — it 
is, as before stated, firmer — it is dead. 

But these fundamental phenomena, the increase of carbonic 
anhydride and the acid reaction, are observable after prolonged 



tetanus. It was, therefore — putting all the facts together that 
we now refer to and others, not forgetting that a muscle is 
always respiring, inhaling oxygen, and exhaling carbonic an- 
hydride — not unreasonable to conclude that normal tetanus 
and rigor mortis were but exaggerated conditions of a natural 
state. The coagulation of the muscle protoplasm (plasma), 
giving rise to myosin, was, however, a serious obstacle to the 
adoption of this view. But it has very recently been urged 
with great plausibility that an old view is correct, viz., that 
7-igor mortis (contracture) is the last act of muscle-life ; it is, in 
fact, a prolonged tetanus or contracture, ending in most cases, 
though not all, in coagulation of the myosin. This state can 
be induced and recovered from in favorable cases by cutting 
oif the blood from a part by ligature, and later readmitting it 
to the starving region. It has been suggested that the prod- 
ucts of the muscle-waste, usually washed away by the blood- 
stream, in such an experiment and after death, collect and act 
as a stimulant to the muscle, causing it to remain in permanent 

The other constituents of dead muscle and their relative 
properties may be learned from the following table (Von Bibra) : 

Water 744-5 

Solids : Myosin, elastic substance, etc., in- 
soluble in water 155'4 

Soluble proteids 19'3 

Gelatin 307 

Extractives and salts 37"1 

Fats '. 23-0 

355 5— 255-5 

Total 1,000 

Among the extractives of muscle very important is creatin 
('2 to "3 per cent), a nitrogenous crystalline body. Certain 
allied forms, as xanthin, hypoxanthin (sarkin), karnin, taurin 
and uric acid, are also found. 

Glycogen (animal starch), very abundant in all the tissues, 
including the muscles of the embryo, is found in smiall quantity 
in the muscles of the adult ; and in the heart-muscle a peculiar 
sugar (inosit) is present. 

It is, of course, very difficult to say to what extent the bodies 
known as extractives exist in living muscle, though that glyco- 
gen, fats, and certain salts are normally present admits of little 


There is a coloring matter in muscle, more abundant in the 
red muscles of certain animals than the pale, allied to haemo- 
globin, if not identical with that body. 

It may be stated as a fact, the exact significance of which 
is unknown, that during contraction the extractives soluble in 
water decrease, while those soluble in alcohol increase. 

It will, however, be very plain, from what has been stated 
in this section, that life processes and chemical changes are 
closely associated, and to realize this is worth much to the 
student of Nature. 

Thermal Changes in the Contracting Muscle. 

Since very marked chemical changes accompany muscular 
contraction, it might be expected that there would be some 
modification in temperature, and probably in the direction of 
elevation. Experiment proves this to be the case. If a ther- 
mometer finely graduated be kept among the muscles of the 
limb of a mammal during the contractions that follow the 
stimulation of the main nerve, a decided rise of temperature 
may be noted during the prolonged tetanus that may be thus 
originated. True, during the contraction of a set of muscles 
under such circumstances, there is a possible fallacy, from the 
excess of blood going to the parts owing to dilatation of the 
blood-vessels, which it would be necessary to exclude — i. e., we 
must either ascertain that such does not take place, or take it 
into account as a factor in the causation of the rise of tempera- 
ture. However, by using a delicate thermopyle, a muscle to 
which no blood passes may be shown to grow warmer during 

But why should a muscle when at rest, as may be shown, 
maintain a certain temperature, unless chemical changes are 
constantly taking place ? As already stated, such is the case, 
and the rise on passing into tetanus is simply an expression of 
increased chemical action. 

What is the nature of the combustion originating this heat ? 
Are certain crude materials withdrawn from the blood and 
burned up directly in the muscle-substance ; or is the muscle 
itself continuously building up and tearing down its own sub- 
stance, all of which implies oxidation ? 

All attempts to explain the facts apart from the latter view 
have been unsuccessful, and we are forced to conclude that 
such is the synoptical statement of the life-history of muscle. 



No macliiiie known to us resembles muscle except super- 
ficially. The steam-engine changes fuel into heat and mechani- 
cal motion, but there the resemblance ends. Muscle changes 
its food, or fuel, not directly into either heat or motion, but into 
itself ; yet as a machine it is more effective than the steam- 
engine, for more work and less heat are the outcome of its 
activity than is the case with the steam-engine. 

The Physiology of Nerve. 

Muscle and nerve are constantly associated functionally, and 
have so much in common that it becomes desirable to study 
them together. Much that has been established for muscle 
holds equally well for nerve ; and the latter, though apparently 
wholly different in structure at first sight, is really not so. 
Nerve has its protoplasmic part (axis-cylinder), which is the 
essential structure, its protective sheaths, and its nuclei (nerve- 

As already indicated, a nerve possesses irritability, and, 
since a muscle does not respond to an electric current sent 
through a nerve except when there is a 
sudden change in the strength of the 
current, it becomes interesting to learn 
why this should be the case. 

Experimental. — In Fig. 190 are shown 
diagrammatically two muscle-nerve prep- 
arations, and the apparatus necessary 
for applying a constant current and a 
(momentary) induced current by single 
shocks to the nerve. 

A strength of current sufiBcient to 
cause a (sub-maximal) contraction by an 
induction shock is determined, and the 
inductorium left at this graduation. A 
constant current of moderate strength is 
allowed to pass into the nerves of the 
preparation. It is found that, in the one 
case, the muscle contraction is increased, 
and in the other diminished or absent,' 
when the same strength of induction 
shock is sent into the nerve at the points 
below the entrance of the constant current — that is to say, 
the irritability of the nerve has been increased or diminished. 

F-iG. IfiO. — Diagrammatic rep- 
resentation of the method 
of testing the excitability 
of the nerve in electrotonus 
(Landois). Positive poles 
marked +, negative, — ; 
the course of current indi- 
cated by arrows. R, Bi, 
i2, Hi., are points at whicn 
exaifebity of the nerve is 
espectaflly altered. 



It is found that when the constant (polarizing) current is pass- 
ing from above downward — that is, when the cathode (nega- 
tive pole) is on the side toward the muscle — the irritability of 
the nerve is increased, and the reverse when the opposite con- 
ditions prevail. 

This altered condition is known as electrotonus. Unfor- 
tunately this term is used somewhat loosely, sometimes being 
employed in the sense now explained; sometimes to denote 
a change of electro-motive force that accompanies the altera- 
tion of irritability ; and agalin to cover all the conditions implied 
in the experiment. It is a fact that during the passage of a 
constant current the natural nerve-current is affected, being 
increased or diminished according to the direction of the polar- 
izing current. There is, however, so much difference of opinion 
in regard to this subject that it is very doubtful whether it 
should be more than noticed in passing. 

But to return to electrotonus, which is both interesting and 
important, it has been found as a result of many experiments 
that profound modifications of the irritability of a nerve do 
take place during the passage of a constant current. These are 
diagrammatically represented in Fig. 191. 

Fig. 191.— Diagrammatjc representation of variations in electrotonus according to strengtli of 
current employed (after Pfliiger). nn\s, section of nerve ; a. anode ( + pole) ; fc, kathode 
(— pole). Curves above the horizontal denote katelectrotonus ; below, the opposite. 

Briefly stated, they are these : 1. The nature of the change 
depends on the direction of the polarizing (constant) current ; 
hence, if the current is descending, there is an increase of irri- 
tability (catelectrotonus) in the portion of the nerve nearest the 
muscle, and vice versa. 2. The extent of the change of irrita^ 
bility is dependent on the strength of the polarizing current. 
3. This change is most marked close to the electrodes, spreads 
to a considerable extent beyond this point without the elec- 
trodes (extra-polar regions), and also exists within the region 
of contact of the electrodes (intra-polar regions). 4. It follows 


that there must be a point at ■wliicli it is not experienced (indif- 
ferent point or neutral point). 

Now, it is possible to understand why a sudden change in 
the current should cause a muscular contraction. An equally- 
sudden change, a profound molecular effect, has been caused, 
and this we must believe essential to the causation of a muscu- 
lar contraction through the influence of a nerve. 

To use an illustration which may serve a good purpose if 
not taken too literally, it is a well-known experience that one 
sitting in a room in which a clock iS ticking soon fails to notice 
this regular sound ; but should the clock stop suddenly or as 
suddenly commence to tick very rapidly, the attention is 
aroused, while a very gradual slowing to cessation or the re- 
verse would have escaped notice. The explanation of such 
facts takes us down to the very foundations of biology ; but 
just now we wish only to elucidate by our own experience 
how it is possible to conceive of a muscle being stimulated 
by the molecular movements of nerve, or rather a change in 

There are important practical aspects to this question. One 
may understand why it is that electricity proves so ready a 
stimulus, and is so valuable a therapeutic agent. It seems, 
in fact, as will be learned later, to be capable of taking the 
place to some extent of that constant nerve influence which 
we believe is being exerted in the higher animals toward 
the maintenance of the regularity of their cell-life (metabol- 

Pathological aad Clinical. — It is believed that in the nerves of 
man, within his living body, the electrotonic condition can be 
induced as in an isolated piece of nerve. Hence, the value of 
the constant current in diminishing nerve irritability in neu- 
ralgia and allied conditions. Apparatus of great nicety of con- 
struction and capable of generating, accurately measuring, and 
conveniently applying electrical currents of different kinds, 
now adds to the resources of the physician. But we are prob- 
ably as yet only on the threshold of electro-therapeutics. 

Law of Contraction (Stimulation). — A given piece of nerve is 
stimulated only by the appearance of catelectrotonus, and the 
disappearance of anelectrotonus ; but the disappearance of cat- 
electrotonus and the appearance of anelectrotonus are without 
effect (Pfluger). This so-called law is supposed to explain the 
following facts, which may be thus expressed in tabular form 
(after Landois) : 






On closing. 

Ou opening. 

On closing. 

On opening. 










R = rest ; C = contraction. 
Electrical Organs. — ^Eleotrical properties can be manifested 
by a large number of fishes ; and the subject is of special 
theoretical interest. It is now established that the development 
of electrical organs points to their 
being specially modified muscles 
— tissues, in fact, in which the 
contractile substance has disap- 
peared and the nervous elements 
become predominant and peculiar. 
No work is done, but the whole of 
the chemical energy is represented 
by electricity. Functionally an 
electric organ (which usually is 
some form of cell, on the walls of 
which nerves are distributed, in- 
closing a gelatinous substance, 
the whole being very suggestive 
of a galvanic battery) closely re- 
sembles a muscle-nerve prepara- 
tion or its equivalent in the nor- 
mal body. The electric organs ex- 
perience fatigue ; have a latent 
period; their discharge is tetanic 
(interrupted) ; is excited by me- 
chanical, thermal, or electrical 
stimuli ; and the efifectiveness of 
the organs is heightened by elevation of temperature, and the 
reverse by cooling, etc. 

FiQ. J93.— The electric - fish torpedo, dis- 
sected to show electric apparatus 
(Huxley). 6, branchiae ; c, brain ; e, 
electric organ ; g, cranium ; me, spinal 
cord ; n, nerves to pectoral flns ; rU, 
nervi laterales; np, branches of pneu- 
mogastric nerves to electric organs ; 
o, eye. 


If during a given period one of two persons raises a weight 
through the same height but twice as frequently as the other, 
it is plain that he does twice the work ; from such a case we 
may deduce the rule for calculating work, viz., to multiply the 
weight aiid height together. 


The effectiveness of a given muscle must, of course, depend 
on the degree to which it shortens, which is from one half to 
three fifths of its length ; and the number of fibers it contains 
— i. e., upon its length and the area of its cross-section, taking 
into account in connection with the first factor the arrange- 
ment of the fibers; those muscles in which the fibers run 
longitudinally being capable of the greatest total shortening. 

There is, as shown by actual experimental trial, a relation 
between the work done and the load to be lifted. With double 
the weight the contraction may be as great as at first, or even 
greater ; but a limit is soon reached beyond which contraction 
is impossible. This principle may be stated thus : The contrac- 
tion is a function of the stimulus, and is illustrated by the 
diagram below (Fig. 193), 

x-rrrT\ \ VTTTT-r-T-r- 

20 30 40 45 60 55 60 65 _ 7b 75 80 90 100 

Fig. 193.— Diagram of muscular contractions with same stimulus and increasing weights. The 
numbers represent grammes (McKendriok). 

It has been shown experimentally that the chemical inter- 
changes in a muscle, acting against a considerable resistance, 
are increased — i. e., the metabolism and the working tension are 

These experimental facts harmonize with our experience 
of a sense of satisfaction and effectiveness in the use of the 
muscles when weights are held in the hands ; and it must be a 
matter of practical importance that each person should, in 
taking systematic exercise, keep to that kind which does not 
either overweight or underweight the muscles. 

Circumstances influencing the Character of Muscular 
AND Nervous Activity. 

The Influence of Blood-Supply. Fatigue. — Fig. 194 shows at a 
glance differences in the curves made by a contracting muscle 
suffering from increasing fatigue. 

Suppose that in such a case the blood had been withheld 
from the muscle, and that it is now admitted, an almost im- 
mediate effect is seen in the nature of the contractions ; but 
even if only saline solution had been sent through the vessels 
of the muscle, a similar change would have been noticeable. 
We may fairly conclude that the blood and saline removed 
something which had been exercising a depressing effect on the 



vitality of the muscle. In a working muscle, like all living 
tissues, there are products of vital actiDn (metabolism) that are 


FiQ. 194.— Curves of a muscle contraction in different stages of fatigue (after Yeo). A, curve 
when muscle was fresh ; fi, C, Z>, E^ each just after muscle had already contracted two 
hundred times. The alteration in length of latent period is not well brought out in these 

poisonous. We have already learned that a working muscle 
generates an excess of carbonic anhydride, and something which 
gives it an acid reaction ; and that it uses up oxygen as well as 
other matters derivable from blood. 

Fatigue will occur, it is well known, if the muscles are used 
for an indefinitely long period, no matter how favorable the 
blood-supply — another evidence that there is, in all probability, 
some chemical product, the result of their own activity, depress- 
ing them ; and this is rendered all the more likely when it is 
learned that the injection of lactic acid, to take one example, 
produces effects like ordinary fatigue. 

It is also a matter of common experience that exercise, while 
beneficial to the whole body, the muscles included, as shown by 
their enlargement under it, becomes injurious when carried to 
the point of fatigue. 

Why the use of the muscles is conducive to their welfare is 
but a part of a larger question. Why does the use of any tissue 
improve it ? 

When the nerve which supplies a muscle is stimulated its 
blood-vessels dilate, and it has been assumed that the same 
happens when a muscle contracts normally in the body; and 
when muscular action is increased there is a corresponding 
augmentation in the quantity of blood driven through the 
muscles in a given period, even if there be no actual increase 
in the caliber of the blood-vessels, for the heart-beat is greatly 

But repose is as necessary as exercise for the greatest effect- 
iveness of the muscles, as the experience of all, and especially 
athletes, proves. 

That the nervous system plays a great part in the nutrition 
of muscles is evident from the fact, among countless others, 
that it is not possible to use the brain to its greatest capacity 
and the muscles to their fullest at the same time ; the individual 


engaged in physical " training " must forego severe mental ap- 
plication. Nervous energy is required for the muscles, and all 
questions of blood-supply are, though important, subordinate. 
But it would be premature to enter into a full discussion of this 
interesting topic now. 

, The sense of fatigue experienced after prolonged muscular 
action is complex, though there can be no doubt that the nerve- 
centers must be taken into account, since any muscular work 
that, from being unusual, requires closer attention and a more 
direct influence of the will, is well known to be more fatiguing. 
On the other hand, the accumulation of products of fatigue 
doubtless reports itself through the local nervous mechanism. 

Separation of Muscle from the Central Nervous System. — "When 
the nerve belonging to a muscle is divided, certain histological 
changes ensue, which may be briefly described as fatty degenera- 
tion, followed by absorption ; and when regeneration of the 
nerve-fibers takes place on apposition of the cut ends, a more 
or less complete restoration of the functions of the nerve fol- 
lows, but the exact nature of the process of repair is not yet 
fully agreed upon ; it seems, in fact, to vary in different cases 
as to details, though it is likely that, in instances in which 
there is a complete return to the normal functionally, the axis- 
cylinders, at all events, are reproduced. 

The degeneration downward is complete ; upward, only to 
the first node of Ranvier. 

Immediately after the section the irritability of the nerve is 
increased, but rapidly disappears, from the center toward the 
periphery (Ritter-Valli law). 

In the mean time the muscle has been suffering. Its irrita- 
bility at first diminishes, then becomes greater than usual to 
shocks from the make or break of the constp^nt current; but 
finally all irritability is lost, and fatty degeneration and disap- 
pearance of true muscular striicture complete the history. It 
is theoretically interesting, as well as of practical importance, 
that degeneration may be delayed by the use of the constant 
current, the significance of which we have already endeavored 
to explain. 

The Influence of Temperature. — If a decapitated frog be placed 
in water of the ordinary temperature, and heat be gradually 
applied, the animal does not move (proving that the spinal cord 
alone is not conscious), but the muscles, when 43° to 60° 0. is 
reached, contract and become rigid, a condition known as " heat- 


There are some advantages in investigating changes in tem- 
perature by the graphic method. Curves from a muscle-nerve 
preparation show that elevation of temperature shortens 'the 
latent period and the curve of contraction. Lowering the tem- 
perature has an effect exactly opposite, as might be sxipposed, 
and these changes take place in the muscles of both cold-blooded 
and warm-blooded animals, though more marked in the latter. 

The modifications evident to the eye are accompanied by 
others, chemical in nature, and a comparison of these shows 
that the rapidity and force of the muscular contraction run 
parallel with the rapidity and extent of the chemical changes. 

Certain drugs also modify the form of the muscle-curve very 
greatly, so that it appears that the molecular action which un- 
derlies all the phenomena of muscle and nerve (for what has 
been said of muscle applies also to nerve, if we substitute 
nervous impulse for contraction) can go on only within those 
narrow bounds which, one realizes more and more in the study 
of physiology, are set to the activities of living things. 

What is the Intimate Nature of Muscular and Nervous Action ? — 
The answers to these questions, to which some allusion has been 
already made, are by no means certain. Some believe that, 
since the nitrogeneous waste of the body, if judged by the urea 
of the urine, is not augmented, some carbohydrate breaks up, 
which would be in accord with the fact that the gaseous inter- 
change of the body generally is increased during exercise, espe- 
cially the excretion of carbonic anhydride. 

Upon the whole, however, such a view does not harmonize 
well with the behavior of protoplasm generally, and it is possi- 
ble to conceive of other processes which would give rise to car- 
bonic anhydride and additional waste products. 

It seems to be' likely that the muscle protoplasm builds up 
and breaks down as a whole ; that this is constantly going on ; 
and that the oxygen which is stored away (intra-molecular) 
sufl&ces for immediate use ; but that when a contraction takes 
place all the chemical processes are heightened, so that we 
may conceive most naturally of the various aspects of muscular 
life as phases of a whole, the parts of which are closely linked 

Another unsettled point is the explanation of the fact that 
a nerve, when stimulated nearer the nerve-center, gives rise to a 
more marked contraction, with the same stimulus than when 
excited nearer the muscle. 

Some suppose that the change that in a nerve constitutes an 


impulse gathers force as it proceeds — tlie avalanche theory of 
Pfliiger ; but it would seem more natural to refer this effect to 
the greater irritability of the nerve nearer the centers. 

The chemistry of dead nerves throws extremely little light 
on the nature of nervous processes. The latter seem, in fact, 
to be accompanied by chemical changes which almost entirely 
elude our methods of detection and estimation. Relatively to 
the chemical the electrical phenomena are predominant; but 
nerve-force is not electrical force, nor are we prepared yet to 
teach that it is the equivalent of that or any other force known 
to us. 

The fact that a nerve maintained in a condition approxi- 
mately normal may be stimulated for hours without exhaus- 
tion, has led some to adopt the tempting conclusion that there 
are no invariable chemical accompaniments of nervous excita- 
tion. But in this and all other instances we think that general 
principles must not be readily set aside by special cases, and 
we should ourselves hesitate to adopt any opinion so contrary 
to all that is known of organic processes as this theory implies, 
except on the amplest and clearest evidence ; and we lay the 
more stress on this, because we think it is a sample of the sort 
of reasoning that is apt to become over-potent with those that 
derive their conclusions wholly or chiefly from laboratory ex- 
periments, to the neglect of wider observations, which put the 
more limited, and possibly more accurate, ones derived from 
the former source, in a truer light, and enable us to establish 
juster relations. 

Unstriped Muscle. 

This form of muscular tissue is characterized by its long 
latent period, its slow wave of contraction, its not passing into 
tetanus, and the progress of the contraction being in either a 
transverse or longitudinal direction, a wave of contraction in 
one cell being capable of setting up a corresponding wave in 
adjoining cells even when no nerve-fibers are distributed to 
them. It is excited, though less readily, by all the kinds of 
stimuli that act upon striped muscle. In the higher groups of 
animals this tissue is chiefly confined to the viscera of the 
chest and abdomen, constituting in the case of some of them 
the greater part of the whole organ. 

The slow but powerful and rhythmical contraction of this 
form of muscle adapts it well to the part such organs play in 


the economy. There are variations, however, in the rapidity, 
force, regularity, and other qualities of the contraction in dif- 
ferent parts : thus, it is comparatively rapid in the iris, and ex- 
tremely powerful and regular in the uterus, serving to produce 
that prolonged yet intermittent pressure essential under the 
circumstances (expulsion of the foetus). 

Comparative. — Muscular contraction is relatively sluggish 
and prolonged among the invertebrates, to which, however, the 
movement of the wings of insects is a marked exception, some 
of them having been shown by the graphic method to vibrate 
some hundreds of times in a second. 

The slow movements of the snail are proverbial. As a rule, 
the strength of the muscles of the invertebrates is incomparably 
greater than that of vertebrates, as witness the powerful grasp 
of a crab's claw or a beetle's jaws. 

These facts are in harmony with the generally slow metab- 
olism of most invertebrates and the lower vertebrates. 

The muscles of the tortoise contract tardily but with great 
power, resist fatigue well, retain their vitality under unfavor- 
able conditions, and after death for a very long period (days). 

Without resorting to elaborate experiments, the student 
may convince himself of the truth of most of the above state- 
ments by observing the movements of a water-snail attached 
to a glass vessel ; the note made by the buzzing of an insect, 
and comparing it with one approaching it in pitch sounded by 
some instrument of music; the force necessary to withdraw 
the foot or tail of a tortoise ; the peristaltic movements of the 
intestine and other organs in a freshly killed animal ; or the 
action of a bee, wasp, or wood-boring beetle on the cork of a 
bottle in which one of them may be inclosed. 

Special Consideeations. 

In the case of weakly (phthisical) persons a sharp tap on 
the chest will often produce a contraction of the muscles thus 
stimulated ; but, in addition, a local contraction lasting some 
little time, known as a wheal or idio-muscular contraction, fol- 
lows. This phenomenon seems to be the result of a special 
irritability in such muscles. 

Cramp may arise under a great variety of circumstances, 
but it seems to be in all cases either a complete prolonged teta- 
nus, in which there is unusual muscular shortening in severe 
cases, at least, or the persistence of a contraction remainder. 


The great differences known to exist between individuals of 
the same species in strength, endurance, fleetness, and other 
particulars in which the muscles are concerned, raise numer- 
ous interesting inquiries. The huild of the greyhound or race- 
horse suggests in itself part of the explanation on mechanical 
principles, lung capacity, etc. But when it is found that one 
dog, horse, deer, or man excels another of the same race in 
swiftness or endurance, and there is nothing in the form to 
furnish a solution, we are prompted to ask whether the muscles 
may not contract more energetically, experience a shortening 
of the latent period, or other phase of contraction; or whether 
they produce less of waste-products or get rid of them more 
rapidly. The whole subject is extremely complicated, and we 
may say here that there is some evidence to show that in races 
of dogs and other animals which surpass their fellows, the 
nerve regulating the heart and lungs (vagus) has greater power ; 
but, leaving tMs and much more out of the account, it is likely 
there are individual differences in the functional nature of the 
muscle. Of equal or more importance is the energizing influ- 
ence of the nervous system, which probably under great excite- 
ment (public boat-races, etc.) acts to produce in man those 
supermaximal contractions which seem to leave the muscle 
long the worse of its unusual action. The nerve-centers, it is 
likely, suffer still more from excessive discharge of nerve-force 
(as we may speak of it for the present) necessary to originate 
the muscular work. Hence the importance of training to 
minimize the non-effective expenditure, ascertain the capacity 
possessed, learn the direction in which weaknesses lie ; and 
equally important the much-neglected period of rest before 
actual contests — if such are to be undertaken at all — so that 
all the activities of the body may gather head, and thus be 
prepared to meet the unusual demand upon them. 

The law of rhythm in organic nature is beautifully illus- 
trated by the behavior of nerve and especially muscle ; at least 
it is more obvious in the case of muscle, at this stage of our 

The regularity with which one phase succeeds another in a 
single contraction ; the essentially rhythmic (vibratory) char- 
acter of tetanus, fatigue and recovery ; the recurrence of in- 
crease and decrease in the muscle and nerve currents — in fact, 
the whole history of muscle is an admirable commentary on 
the truth of the law of rhythm, into which in further detail 
space will not permit us to enter. 


It is a remarkable fact that tlie endurance of man, especially 
civilized man, seems to be greater than that of any other mam- 
mal. It may be hazardous to express a dogmatic opinion as to 
the reason of this, but the influence of the mind over the body 
is unquestionably greater in man than in any other animal; 
and, if we are correct in assigning so much importance to the 
influence of the nervous system in maintaining the proper 
molecular balance which is at the foundation of the highest 
good of an organism, we certainly think that it is in this direc- 
tion we must look for the explanation of the above-mentioned 
fact, and much more that would otherwise be obscure in man's 
functional life. 

Functional Variations. — We have endeavored, in treating this 
subject of muscle, to point out how the phenomena vary with 
the animal, the kind of muscle, and the circumstances under 
which they are manifested. It may be shown that every one 
of the qualities which a muscle possesses, varies with the tem- 
perature, the blood-supply, the duration of its action, the char- 
acter of the stimulus, and other modifying agents. Not only 
are there great variations for different groups of animals, but 
lesser ones for individuals ; though the latter are made more 
evident indirectly than when tested by the usual laboratory 
methods ; but they must be taken account of if we would un- 
derstand animals as they are. Some of these will be referred 
to later. 

If a muscle-cell be regarded in the aspect that we are now 
emphasizing, its study will tend to impress those fundamental 
biological laws, the comprehension of which is of more impor- 
tance than the acquisition of any number of facts, which, how- 
ever interesting, can, when isolated, profit little. 

Experiment has not done much directly, and it seems can 
not at present, for the physiology of man, though more may be 
accomplished as regards muscle and nerve than some other 
tissues. It is, of course, possible to measure the rapidity of 
the passage of a nervous impulse and to study electrical phe- 
nomena generally to some extent. Putting all that is known 
together, it would appear that,, without referring to minor dif- 
ferences which unquestionably exist, the muscle and nerve 
physiology of man corresponds pretty closely with that of one 
of the highest mkmmals, and, as compared with the lower ver- 
tebrates, his muscles and nerves possess an irritability of a 
very exalted type, with, however, a corresponding loss or dif- 
ference in other directions. 


Stimmary of the Physiology of Muscle and Werve. — The move- 
ments of a muscle are distinguished from those of other forms 
of protoplasm by their marked definiteness and limitation. 

The contraction of a muscle-fiber (cell) results in an increase 
in its short transverse diameter, and a diminution of its long 
diameter, without appreciable change in its total bulk. 

Muscle and nerve are not automatic, but are irritable. 
Though muscle normally receives its stimulus through a nerve, 
it possesses independent irritability. 

Stimuli may be mechanical, chemical, thermal, electrical, and 
in the case of muscle, nervous ; and to be effective they must 
be applied suddenly and last for a brief but appreciable time. 

Electrical stimulation, especially, is only effective when 
there is a sudden change in the force or direction of the cur- 
rents. This applies to both muscle and nerve. 

A muscular contraction consists of three phases : the latent 
period, the period of rising, and the period of falling energy, 
or of contraction and relaxation. 

When the phase of relaxation is minimal and that of con- 
traction approaches continuity, a tetanus results. The contrac- 
tions of the muscles in situ are tetanic, and are accompanied 
by a low sound, evidence in itself of their vibratory character. 

The prolonged contraction of a muscle leads to fatigue; 
owing in part, at least, to the accumulation of waste-products 
within the muscle which depress its energies. 

This is a necessary consequence of the fact that all proto- 
plasmic activity is accompanied by chemical change, and that 
some of these processes result in the formation of products 
which are hurtful and are usually rapidly expelled. 

Muscular contraction is accompanied by chemical changes, 
in which the formation of carbon dioxide, and some substance 
that causes an acid reaction to take the place of an alkaline or 
neutral one. Since free oxygen is not required for the act of 
contraction, but is still used up by a contracting muscle, it may 
be assumed that the oxygen that plays a part in actual con- 
traction is intra-molecular. 

Chemical changes are inseparable from the vital processes 
of all protoplasm, and the phenomena of muscle show that 
they are constantly in operation, but exalted during ordinary 
contraction and that tetanic condition which precedes and 
may end in coagulation of muscle plasma and the formation of 
myosin. The latter is a result of the disorganization of muscle, 
and has points of resemblance to the coagulation of the blood. 


The contraction of a muscle, and the passage of a nervous 
impulse, are accompanied by electrical changes. Whether cur- 
rents exist in uninjured muscle and nerve is a matter of con- 
troversy. All physiologists agree that they exist in muscle 
(and nerve) during functional activity. This electrical condi- 
tion is termed the " negative variation " by those believing in 
currents of rest, and the " current of action " by those holding 
opposite opinions. The current is of momentary duration, and 
is manifested during the latent period of muscle, in which also 
the chemical changes take place ; so that a muscular contrac- 
tion must be regarded as the outcome of the events of the 
latent period, which is, therefore, though the shortest, the most 
important of the phases of a muscular contraction. 

During the passage of a constant (polarizing) current from 
a battery through a nerve, it undergoes a change in its irrita- 
bility and shows a variation in the electro-motive force of the 
ordinary nerve-current (electrotonus). This fact is of thera- 
peutic importance. The electrical phenomena of nerve are alto- 
gether more prominent than the chemical, the reverse of which 
is true of muscle. The activity of a muscle (and nerve proba- 
bly) is accompanied by the generation of heat, an exaltation of 
which takes place during muscular contraction. 

Rigor tnortis causes an increase in temperature and the 
chemical interchanges which accompany the other phenomena. 
A muscle may also become rigid by passing into rigor caloris. 
Living muscle is translucent, alkaline or neutral in reaction, 
and elastic ; dead muscle, opaque, acid in reaction, and devoid 
of elasticity, but firmer than living muscle, owing to coagula- 
tion of the muscle-plasma. Dead nerve undergoes similar 

The elasticity of muscle is restricted but perfect within its 
own limits. It differs from that of inorganic bodies in that the 
increments of extension are not directly proportional to the in- 
crements of the weight. When overstretched, muscle does not 
return to its original length^ (loss of elasticity), hence the serious 
nature of sprains. 

It is important to regard muscular elasticity as an expres- 
sion of vital properties. 

The work done by a muscle is ascertained by multiplying 
the load lifted by the height; and the capacity of an individual 
muscle will vary with its length, the arrangement of its fibers, 
and the area of its cross-section (i. e., on the number of fibers). 

The work done may be regarded as a function of the resist- 



ance (load), as the contraction is also a function of the stimulus. 
The separation of a muscle from its nerye by section of the lat- 
ter leads to certain changes, most rapid in the nerve, which 
show that the two are so related that prolonged independent 
vitality of the muscle is impossible, and make it highly proba- 
ble that muscle is constantly receiving some beneficial stimulus 
from nerve, which is exalted and manifest when contraction 
takes place. 

The study of the development of the electrical cells of cer- 
tain fishes shows that they are greatly modified muscles in 
which contractility, etc., has been exchanged for a very decided 
exaltation of electrical properties. It is likely, though not 
demonstrated, that all forms of protoplasm undergo electrical 
changes— that these> in fact, like chemical phenomena, are vital 

The phases of the contraction of smooth muscular tissue are 
all of longer duration ; the contraction- wave passes in different 
directions, and may spread into cells devoid of nerves, which 
we think not unlikely also to be the case, though less so, for all 
forms of muscle. 

The smooth muscle-cell must be regarded as a more primi- 
tive, less specialized, form of tissue. Variations in all the phe- 
nomena of muscle with the animal and the circumstances are 
clear and impressive. Finally, muscle illustrates an evolution 
of structure and function, and the law of rhythm. 


Since in the higher vertebrates the nervous system is domi- 
nant, regulating apparently every process in the organism, it 
will be well before proceeding further to treat of some of its 
functions in a general way to a greater extent than we have yet 

Manifestly it must be highly important that an animal shall 
be able to place itself so in relation to its surroundings that it 
may adapt itself to them. Prominent among these adaptations 
are certain movements by which food is secured and dangers 
avoided. The movements having a central origin, a peripheral 
mechanism of some kind must exist so as to place the centers 
in connection with the outer world. Passing by the evolution 
of the nervous system for the present, it is found that in verte- 
brates generally there is externally a modification of the epi- 


thelial covering of the body (end-organ) in whicli a nerve ter- 
minates, wMda latter may be traced to a cell or cells removed 
from the surface {center), and from ■wbich in most cases other 
nerves proceed. 

The nervous system, we may remind the student, consists in 
vertebrates of centers in which nerve-cells abound, united by 
nerve-fibers and by the most delicate form of connective tissue 
known, in connection with which there are incased strands of 
protoplasm or nerves as outgrowths. The main centers are, of 
course, aggregated in the brain and spinal cord. 

It is possible to conceive of the work of a nervous system 
carried on by a single cell and an afferent and efferent nerve ; 
but inasmuch as such an arrangement would imply that the 
central cell should act the part of both receiving and origi- 
nating impulses (except it were a mere conductor, in which case 
there would be no advantage whatever in the existence of a cell 
at all), according to the principle of the physiological division 
of labor, we might expect that there would be at least two cen- 
tral cells — one to receive and the other to transmit impulses — 
or at least that there should be some specialization among the 
central cells; and we shall have good reason later to believe 
that this has reached a surprising degree in the highest ani- 

Moreover it would be a great advantage if the termination 
of the ingoing (afferent) nerve should not lie exposed on the 
surface, but be protected by some form of cell that had also the 
power to transmit to it the impressions received from without, 
in a form suitable to the nature of the nerve and the needs of 
the organism. 

So that a complete mechanism in its simplest form would 
furnish : 1. A peripheral cell or nerve end-organ. 2. An affer- 
ent or sensory nerve. 3. Two or more central cells. 4. An 
efferent nerve, usually connected with — 5. A muscle or other 
form of cell, the action of which may be modified by the out- 
going nerve, or, as we should prefer to say, by the central nerv- 
ous cells through the efferent nerve. The advantages of the 
principal cells being within and protected are obvious. 

When, then, an impression made on the peripheral cell is 
carried inward, there modified, and results in an outgoing nerv- 
ous impulse answering to the afferent one, giving rise to a mus- 
cular contraction or other effect not confined to the recipient 
cells, the process is termed reflex action. 

The great size, the multiplicity of forms, the distinct out- 


line and large nuclei of nerve-cells, suggest the probability 
that they play a very important part, and such is found to be 
the case. Indeed, in some sense the rest of the nervous system 
may be said to exist for them. 

Probably nerve-cells do sometimes act as mere conductors 
of nervous impulses originating elsewhere, but such is their 
lowest function. Accordingly, it is found that the nature of 
any reflex action depends most of all on the behavior of the 
central cells. 

It can not be too well borne in mind that nerves are con- 
ductors and such only. . They never originate impulses. 

The properties considered in the last chapter are common to 
all kinds "of nerves known ; and though we must conceive that 
there are some differences in the form of impulses, these are to 
be traced, not to the nerve primarily, but to the organ in which 
it ends peripherally or to the central cells. 

To return to reflex action, it is found that the muscular re- 
sponse to a peripheral irritation varies with the point stimu- 
lated, the intensity of the stimulus, etc., but is, above all, de- 
termined by the central cells. 

Nerve influence may be considered as following lines of 
least resistance, and there is much evidence to show that an 
impulse having once taken a certain path, it is easier for it to 
pass in this direction a second time, so that we have the founda- 
tion of the laws of habit and a host of interesting phenomena 
in this simple principle. 

It is found that, in a frog deprived of its brain and sus- 
pended by the under jaw, there is no movement unless some 
stimulus be applied ; but if this be done under suitable condi- 
tions, instructive results follow, which we now proceed to indi- 
cate briefly. The experiments are of a simple character, which 
any student may carry out for himself. 

Experimental. — Preparing a frog by cutting off the whole 
of the upper jaw and brain-case after momentary anaesthesia, 
suspend the animal by the lower jaw and wait till it is perfect- 
ly quiet. Add to water in a beaker sulphuric acid till it tastes 
distinctly but not strongly sour, to be used as a stimulus. 1. 
Apply a small piece of bibulous paper, moistened with the acid, 
to the inner part of the thigh of the animal. The leg will be 
drawn up and the paper probably removed. Eemove the paper 
and cleanse the spot. 2. Apply a similar piece of paper to the 
middle of the abdomen ; one or both legs will probably be 
drawn up, and wipe off the offending body. 3. Let the foot of 


the frog hang in the liquid; after a few moments it will be 
withdrawn. 4. Repeat, holding the leg ; probably the other leg 
will be drawn up. 5. Apply stronger acid to the inside of the 
right thigh ; the whole frog may be convulsed, or the left leg 
may be put in action after the right. Even if the stimulating 
paper be applied near the anus, it will be removed by the hind- 
legs. 6. Beneath the skin of the back (posterior lymph-sac) 
inject a few drops of liquor strychnise of the pharmacopoeia ; 
after a few minutes apply the same sort of stimulus to the 
thigh as before. The effects follow more quickly and are 






Fig. 195.— Diagram intended to illustrate nervoiis mechanism of— 1, automatism ; 2, reflex 
action ; and 3, how nervous impulses in the latter case ma^ i)ass into the higher parts of 
brain and become part of consciousness, or be wholly inhibited. A reflex or autoniatic 
center may for the sake of simplicity be reduced to a single cell, as above on the left. 

much more marked — the animal, it may be, passing into a gen- 
eral tetanic spasm. 

These experiments may be varied, but suffice to establish 
the following conclusions : 1. The stimulus is not immediate- 
ly effective, but requires to act for a certain variable period, 
depending chiefly on the condition of the central nervous sys- 
tem. 2. The movements of the muscles harmonize (are co-ordi- 
nated), and tend to accomplish some end — are purposive. If 


tlie nerve alone and not the skin be stimulated, there may be a 
spasm only and not adaptive movement. 3. Nervous impulses, 
"when very abundant, may pass along unaccustomed or less ac- 
customed paths (experiments 4 and 5). This is sometimes spoken 
of as the rg,diation of nervous impulses. 

The sixth experiment is very important, for it shows that 
the result varies far more with the condition of the nervous 
centers (cells) than the stimulus, the part excited, or any other 

Automatism. — But, seeing that these central cells have such 
independence and controlling power, the question arises. Are 
these, or are there any such cells, capable of originating im- 
pulses in nerves wholly independent of any stimulus from 
without ? In other words, have the nerve-centers any true 
automatism ? Apparently this quality is manifested by uni- 
cellular organisms of the rank of Amoeba. Has it been lost, 
or has it become a special characteristic developed to a high 
degree in nerve-cells ? 

We shall present the facts and the opinions based on them 
as held by the majority of physiologists, reserving our own 
criticisms for another occasion : 1. The medulla oblongata is 
supposed to be the seat .of numerous small groups of cells, to a 
large extent independent of each other, that are constantly 
sending out nervous impulses which, proceeding to certain sets 
of muscles, maintain them in rhythmical action. One of the 
best known of these centers is the respiratory. 2. The poste- 
rior lymph hearts of the frog are supplied by nerves (tenth 
pair), which are connected, of course, with the spinal cord. 
When these nerves are cut, the hearts for a time cease to beat, 
but later resume their action. 3. The heart beats after all its 
nerves are cut, and it is removed from the body, for many hours, 
in cold-blooded animals. 4. The contractions of the intestine 
take place in the absence of food, and in an isolated piece of 
the gut. The intestine, it will be remembered, is abundantly 
supplied with nerve-elements. 5. In a portion of the ureters, 
from which it is Relieved- nerve-cells are absent, rhythmical 
action takes place. 

Conclusions. — 1. Whether the action of the respiratory and 
similar centers could continiie in the absence of all stimuli can 
not be considered as determined. 2. That there are regular 
rhythmical discharges from the spinal nerve-cells along the 
nerves to the lymph hearts seems also doubtful. 3. Later in- 
vestigations render the automaticity of the heart more uncer- 


tain than ever, so that the result stated above (3) must not be 
interpreted too rigidly. 

Similar doubts hang about the other cases of apparent au- 

As regards the various comparatively isolated collections of 
cells known as ganglia, the evidence, so far as it goes, is against 
their possessing either automatic or reflex action; and new 
views of their nature will be presented in due course. 

Nervous Inhibition. — If the pneumogastric nerve passing 
from the medulla to the heart of vertebrates be divided and 
the lower (peripheral) end stimulated, a decided change in the 
action of the heart fqllows, which may be in the direction of 
weakening or slowing, or positive arrest of its action. 

Assuming, for the present, that the cells (center) of the me- 
dulla have the power to bring about the same result, it is seen 
that such nervous influence is preventive or inhibitory of the 
normal cardiac beat, ^o that the vagus is termed an inhibitory 
nerve. Such inhibition plays a very important part in the 
economy of the higher animals, as will become more and more 
evident as we proceed. The nature of the influences that pro- 
duce such remarkable results will be discussed when we treat 
of the heart. 

An illustration will probably serve in the mean time to make 
the meaning of what has been presented in this chapter more 
clear and readily grasped. 

In the management of railroads a very great variety of 
complicated results are brought about, owing to system and 
orderly arrangement, by which the wishes of the chief mana- 
ger are carried out. 

Telegraphing is of necessity extensively employed. Sup- 
pose a message to be conveyed from one ofBce to another, this 
may (1) simply pass through an intermediate ofl6.ce, without 
special cognizance from the operator in charge ; (3) the operator 
may receive and transmit it unaltered ; (3) he may be required 
to send a message that shall vary from the one he receives in a 
greater or less degree ; or (4) he may arrest the command alto- 
gether, owing to the facts which he alone knows and upon 
which he is empowered always to act according to his best dis- 

In the first instance, we have an analogy with the passage 
of a nervous impulse through central fibers, or, at all events, 
unaffected by cells ; in the second, the resemblance is to cells 
acting as conductors merely ; in the third, to the usual behavior 


oif the cells in reflex action ; and, in the fourth, we have an in- 
stance of inhibition. The latter may also be rendered clear by 
the case of a horse and its rider. The horse is controlled by the 
rider, who may be compared to the center, through the reins 
answering to the nerves, though it is not possible for either rider 
or reins to originate the movements of the animal, except as 
they may be stimuli, which latter are only effective when there 
are suitable conditions — when, in fact, the subject is irritable 
in the physiological sense. 


Every tissue, every cell, requiring constant nourishment, 
some means must necessarily have been provided for the con- 
veyance of the blood to all parts of the organism. We now 
enter upon the consideration of the mechanisms by which this 
is accomplished and the method of their regulation. 

Let us consider possible mechanisms, and then inquire into 
their defects and the extent to which they are found embodied 
in nature. 

That there must be a central pump of some kind is evident. 
Assume that it is one-chambered, and with an outflow-pipe 
which is continued to form an inflow-pipe. This might be pro- 
vided with valves at the openings, by which energy would be 
saved by the prevention of regurgitation. In such a system 
things must go from bad to worse, as the tissues, by constantly 
using up the prepared material of the blood, and adding to it 
their waste products, would effect their own gradual starvation 
and poisoning. 

It might be conceived, however, that waste at all events was 
got rid of by the blood being conducted through some elimi- 
nating organs ; and assume that one such at least is set aside 
for respiratory work. If the blood in its course anywhere 
passed through such organs, the end would be attained in some 
degree ; but if the division of labor were considerable, we should 
suppose that, gaseous interchange being so very important as 
we have been led to see from the study of the chapters on gen- 
eral biology, and on muscle, organs to accomplish this work 
might receive the blood in due "course and return it to the cen- 
tral pump in a condition eminently fit from a respiratory point 
of view. 

Such, however, would necessarily be associated with a more 


complicated pump ; and, if this were so constructed as to pre- 
vent the mixture of blood of different degrees of functional 
value, higher ends would be attained. 

Turning to the channels themselves in which the blood 
flows, a little consideration will convince one that rigid tubes 
are wholly unfit for the purpose. Somewhere in the course of 
the circulation the blood must flow sufficiently slowly, and 
through vessels thin enough to permit of that interchange be- 
tween the blood and the tissues, through the medium of the 
lymph, which is essential from every point of view. The main 
vessels must have a strength sufficient to resist the force with 
which the blood is driven into them. 

Now, it is possible to conceive of this being accomplished 
with an intermittent flow ; but manifestly it would be a great 
advantage, from a nutritive aspect, that the flow and therefore 
the supply of tissue pabulum be constant. With a pump regu- 
larly intermittent in action, provided with valves, elastic tubes 
having -a resistance in them somewhere sufficient to keep them 
constantly over-distended, and a collection of small vessels with 
walls of extreme thinness, in which the blood-current is great- 
ly slackened, a steady blood-flow would be maintained, as the 
student may readily convince himself, by a few experiments of 
a very simple kind : 

1. To show the difference between rigid tubes and elastic 
ones, let a piece of glass rod, drawn out at one end to a small 
diameter, have attached to the other end a Higginson's (two- 
bulb) syringe, communicating with a vessel containing water. 
Every time the bulb is squeezed, water flows from the end of 
the glass rod, but the outflow is perfectly intermittent. 

2. On the other hand, with a long elastic tube of India-rub- 
ber, ending in a piece of glass rod drawn out to a point as be- 
fore, if the action of the pump (bulb) be rapid the outflow will 
be continuous. An apparatus that every practitioner of medi- 
cine requires to use answers perhaps still better to illustrate 
these and other principles of the circulation, such as the pulse, 
the influence of the force and frequency of the heart-beat on the 
blood-pressure, etc. We refer to a two-bulb atomizer, the bulb 
nearer the outflow serving to maintain a constant air-pressure. 

We may now examine the most perfect form of heart 
known, that of the mammal, in order to ascertain how far it 
and its adjunct tubes answer to a priori expectations. 

The Mammalian Heart. — In order that the student may gain 
a correct and thorough knowledge of the anatomy of the heart 



and the working of its various parts, we recommend him to 
pursue some such course as the following : 

1. To consult a number of plates, such as are usually fur- 
nished in works on anatomy, in order to ascertain in a general 
way the relations of the heart to other organs, and to the chest 
wall, as well as to become familiar with its own structure. 

3. To supplement this with reading the anatomical descrip- 
tions, without too great attention to details at first, but with 
the object of getting his ideas clear so far as they go. 

3. Then, with plates and descriptions before him, to examine 
several dead specimens of the heart of the sheep, ox, pig, or 
other mammal, first somewhat generally, then systematically, 
with the purpose of getting a more exact knowledge of the 

various structures and 
their anatomical as well 
as physiological relations. 
We would not have 
the student confine his 
attention to any single 
form of heart, for each 
shows some one structure 
better than the others; 
and the additional advan- 
tages of comparison are 
very great. The heart of 
the ox, from its size, is 
excellent for the study of 
valvular action, and the 
framework with which 
the muscles, valves, and 
vessels are connected ; 
while the heart of the pig 
(and dog) resemble the 
human organ more close- 
ly than most others that 
can be obtained. 

It will be found very 

helpful to perform some 

of the dissections under 

and by the use of 

. , or some other fluid 

left ventricle; 4, portion of the same with papillary ,, ,. /> in i 

muscle attached i 5, 5', the other papillary raus- the aCtlOn OI the VaiveS 

cles ; 6, one segment of the mitral valve : 7. in t_ i j • i. 

aorta is placed over the semilunar valves. may De learned. aS it Can 

Tin. 196.— The left auricle and ventricle opened and 



in no other way. By a little manipulation the heart may be so 
held that water may be poured into the orifices, prepared by a 
removal of a portion of the blood-vessels or the auricles, when 
the valves may be seen closing together, and thus revealing 
their action in a way which no verbal or pictorial representa- 
tions can do at all adequately. 

Fio. 197.— View of the orifices of the heart from below, the whole of the ventricles having 
been cut away (after Huxley). RAV^ right auriculo- ventricular orifice, surrounded by 
the three fiaps, t. v. 1, t. v. 2, t. v. 3, of the tricuspid valve, which are stretched by weights 
attached to the cAordcE icndmece. i/.4F, left auriculo-ventricular orifice, etc. Pj4, orifice 
of the pulmonary artery, the semilunar valves represented as having met and closed 
together. AO, orifice of the aorta. 

A heart thoroughly boiled and allowed to get cold shows, 
on being pulled somewhat apart, the course, attachment, and 
other features of the fibers very well, as also the skeleton of 
the organ, which may be readily separated. 

When this has all been done, the half is not yet accom- 
plished. A visit to an abattoir will now repay amply for the 
time spent. Animals are there killed and eviscerated so rapidly 
that an observer may not only gain a good practical acquaint- 
ance with the relations of the heart to other parts, but may 
often see the organ still living and exemplifying that action 
peculiar to it as it gradually approaches quiescence and death 
— a matter of the utmost importance. 

If the student will then compare what he has learned of the 
mammalian heart in this way with the behavior of the heart 
of a frog, snake, fish, turtle, or other animal that may be killed 
after brief ether narcosis, without cessation of the heart's ac- 



tion, he will have a broader basis for his cardiac physiology 
than is usual ; and we think we may promise the medical stu- 
dent, who will in this 
m ^..-jTO^.-^^-^ and other ways that 

may occur to him 
supplement the usual 
work on the human 
cadaver, a pleasure 
and profit in the 
study of heart - dis- 
ease which come in 
no other way. 

"With the view of 
assisting the obser- 
vation of the student 
as regards the heart 
of the mammal, we 
would call special at- 
tention to the follow- 
ing points among 
others : Its method of 
suspension, chiefly by 
its great vefssels ; the 
strong fibrous frame- 
work for the attachment of valves, vessels, and muscle-fibers ; 
the great complexity of the arrangement of the latter; the 
various lengths, mode of attachment, and the strength of the 
inelastic chordae tendinese ; the papillary muscles which doubt- 
less act at the moment the valves flap back, thus preventing 
the latter being carried too far toward the auricles, the pocket- 
ing action of the semilunar valves, with their strong margin 
and meeting nodules {corpora aurantii) ; the relative thickness 
of auricles and ventricles, and the much greater thickness of 
the walls of the left than of the right ventricle — differences 
which are related to the work these parts perform. 

The latter may be well seen by making transverse sections 
of the heart of an animal, especially one that has been bled to 
death, which specimen also shows how the contraction of the 
heart obliterates the ventricular cavity. 

It will also be well worth while to follow up the course of 
the coronary arteries, noting especially their point of origin. 

The examination of the valves of the smaller hearts of cold- 
blooded animals is a matter of greater difficulty and is facili- 

FiQ. 198.— Orifices of the heart seen from above, after the 
auricles and great vessels had been cut away (after 
Huxley). PA, pulmonary artery, with its semilunar 
valves. Ao, aorta in a similar condition. JRAV, right 
auriculo - ventricular orifice, with m. v. 1 and 2 flaps 
of mitral valve ; 6, style passed into coronary vem. 
On the left part of LAV the section of the auricle is 
carried through the auricular appendage^ hence the 
toothed appearance due to the portions m relief cut 



bated by dissection under water with the help of a lens or dis- 
secting microscope ; but even without these instruments much 
may be learned, and certainly that the valves are relatively to 
those of the mammalian heart imperfectly developed, will be- 
come very clear. 

Circulation op the Blood in the Mammal. 

It is highly important and quite possible in studying the 
circulation to form a series of mental pictures of what is trans- 
piring. It will be borne iu' mind that there is a set of elastic 
tubes of relatively thick walls, standing open when cut across, 
dividing into smaller and smaller branches, and finally ending 
in vessels of more than cobweb fineness, and opening out into 
others, that become larger and larger and fewer and fewer, till 
they are gathered up into two of great size which form the right 
auricle. The larger pipes consist 
everywhere of elastic tissue prop- 
er, muscular tissue (itself elas- 
tic), fibrous tissue, and a flat epi- 
thelial lining, so smooth that the 
friction therefrom must be mini- 
mal as the blood flows over it. 

The return tubes or veins are 
like the arteries, but so thin that 
their walls fall together when cut 
across. They are different from 
all the other blood-tubes in that 
they possess valves opening to- 
ward the heart throughout their 
course. The veins are at least 
twice as numerous as the arte- 
ries, and their capacity many 
times greater. The small vessels 
or capillaries are so abundant 
and wide-spread that, as is well 
known, the smallest cut any- 
where gives rise to a flow of blood, 
owing to section of some of these 
tubes, which, it will be remem- 
bered, are not visible to the un- 
aided eye. It is estimated that their united area is several 
hundred (500 to 800) times that of the arteries. 

Fio. 199.— Various layers of the wsiUs of a 
small artery (Landois). e, endothelium ; 
i. e, internal elastic lamina ; c. m, circu- 
lar muscular fibers of the middle coat ; 
c. f, connective tissue of the outer coat, 
or T. adventitia. 



If we suppose the epithelial lining pushed out of a small 
artery we have, so far as structure alone goes, a good idea of a 
capillary — i. e,, its walls are but one cell thick, and these cells 

Fia. 200. Fig. 201. 

Fig. 200. — Vein with valves lying open (Dalton), 

Fig. 201.— Vein with valves closed, the blood passing on by a lateral branch below (Dalton). 

though long are extremely thin, so that it is quite easy to un- 
derstand how it is that the amcsboid corpuscles can, under cer- 

FiB. 202.— Capillary blood-vessels (.Landois). The cement-substance between the endothelium- 
has been rendered dark by silver nitrate, and the nuclei made prominent by staining. 

tain circumstances, push their way through its probably semi- 
fluid walls. 

From what has been said, it will be seen that the whole col- 
lection of vascular tubes may be compared to two inverted fun- 



Fig. 203.— Diagram to illustrate the relative proportions of the aggregate Bectional area of the 
different parts of the vascular system (after Yeo). A, aorta ; C, capillaries ; V, veins. 

nels or cones "with the smaller end toward the heart and the 
widest portions representing the capillaries. 

The Action of the Mammalian Heart. 

Very briefly what takes place may be thus stated : The 
right auricle contracting squeezes the blood through the au- 
riculo-ventricular opening into the right ventricle, never quite 
emptying itself probably ; immediately after the right ventricle 
contracts, by which its valves are brought into sudden tension 
and apposition, thus preventing reflux into the auricle ; while 
the blood within it takes the path of least resistance, and the 
only one open to it into the pulmonary artery, and by its 
branches is conveyed to the capillaries of the lungs, from 
which it is returned freed from much of its carbonic anhy- 
dride and replenished with oxygen, to the left auricle, whence 
it proceeds in a similar manner into the great arterial main, 
the aorta, for general distribution throughout the smaller 
arteries and the capillaries to the most remote as well as the 
nearest parts, from which it is gathered up by the veins and 
returned laden with many impurities, and robbed of a laifge 
proportion of its useful matters, to the right side of the hearL 

It will be remembered that corresponding subdivisions of 
each side of the heart act simultaneously, and that any decided 



departure from this harmony of rhythm -would lead to serious 

Capillaries of the 
Head, etc. 

Superior Vena Cava. 

Inferior Vena Cava. 

Capillaries of Liver. 
Portal Vein. 

Pulmonary Capil- 

Main Arterial Trunk. 

Capillaries of 
Splanchnic Area. 

Capillaries of Trunk 
and Lower Ex- 

Fig. 204.— Diagram of the circulation. The arrows indicate the course of the blood. Though the 
pulmonary and the upper and the lower parts of the systemic circidation are represented 
so as to show the distinctness of each, it will be also apparent that they are not independ- 
ent. Relative size of different parts of the system is only very generally indicated. 

The Velocity op the Blood and Blood-Pressure. 

If the relative capacity and arrangement of the various 
parts of the circulatory system be as has been represented, it 
follows that we may predict with some confidence, apart from 
experiment, what the speed of the flow and the vascular ten- 
sion must be in different parts of the course of the circulation. 

We should suppose that, in the nature of the case, the ve- 
locity would be greatest in the large arteries, gradually dimin- 
ish to the capillaries, in which it would be much the slowest, 
and, getting by degrees faster, would reach a speed in the largest 
veins approaching that of the corresponding arteries. 


The methods of determining the velocity of the blood-stream 
have not entirely surmounted the difficulties, but they do give 
results in harmony with the above-noted anticipations. 

The area of the great aortic trunk being so much less than 
that of the capillaries, the flow in that vessel we should expect to 
be very much swifter than in the arterioles or the capillaries. 
Moreover, there must be a great differance in the velocity during 
cardiac systole and diastole, and according as the beat of the 
heart is forcible or otherwise. But, apart from these more ob- 
vious differences, there are variations depending on complex 
changes in the peripheral circulation, owing to the frequent 
variations in the diameter of the arterioles in difiEerent parts, 
as well as differences in the resistance offered by the capillaries, 
the causes of which are but ill understood, though less obscure, 
we think, than they are often represented to be. Since, for the 
maintenance of the circulation, the quantity of blood enter- 
ing and leaving the heart must be equal, in consequence of the 
sectional area of the great veins that enter the heart being 
greater than that of the aorta, it follows that the venous flow 
,even at its quickest is necessarily slower than the arterial. 

Comparative. — There must be great variations in velocity in 
different animals, as such measurements as have been made 
demonstrate. Thus, in the carotid of the horse, the speed of 
the blood-current is calculated as about 306 mm., in the dog at 
from 205 to 357 mm. These results can not be considered as 
more than fair approximations. 

Highly important is it to note that the rate of flow in the 
capillaries of all animals is very slow indeed, not being as much 
as 1 mm. in a second in the larger mammals. The time occu- 
pied by the circulation is also, of course, variable, being as a 
rule shorter the smaller the animal. As the result of a num- 
ber of calculations, though by methods that are more or less 
faulty, the following law may be laid down as meeting approxi- 
mately the facts so far as warm-blooded animals are concerned : 

The circulation is effected by 27 heart-beats; thus, for a 
man with a pulse of 81, the time occupied in the completion of 
the course of the blood from and to the heart would be f^- = 3 ; 
i. e., the circulation is completed three times in one minute, or 
its period is twenty seconds ; and it is to be well borne in mind 
that by far the greater part of this time is occupied in travers- 
ing the capillaries. 




The Cieculation under the Microscope. 

There are few pictures more instructive and impressive than 
a view of the circulation of the blood under the microscope. 
It is well to have similar preparations, one under a low power 
and another under a magnification of 300 to 500 diameters. With 
the former a view of arterioles, veins, and capillaries may he 

Fig. 805.- Portion of the web of a frog's foot as seen imder a low magnifying power, sliowing 
the blood-vesselsi and in one corner the pigment-spots (after Huxley), a, small arteries 
(arterioles); v, siliall veins. The smaller vessels are the capillaries. The com^e of the 
blood is indicated by arrows. 

obtained at once. Many different parts of animals may be used, 
as the web of the frog's foot, its tongue, lung, or mesentery ; 
the gill or tail of a small fish, tadpole, etc. 

The relative size of the vessels ; the speed of the blood-flow ; 
the greater velocity of the central part of the stream ; the aggre- 
gation of colotless corpuscles at the sides of the vessels, and the 
occasional passage of one through a capillary wall, when the 
exposure has lasted some time ; the crowding of the red cells ; 
their plasticity ; the small size of some of the capillaries, barely 



allowing the corpuscles to be squeezed through. ; the changes in 
the velocity of the current, especially in the capillaries ; its pos- 
sible arrest or retrocession; the velocity in one so much greater 
than in its neighbor, without very obvious cause — all this and 
much more forms, as we have said, a remarkable lesson for the 
thinking student. This, like all microscopic views, especially 
if motion is represented, has its fallacies. It is to be remem- 

FiG. 206.— Circulation in the web of the frog's foot (Wagner). V, venous trunk composed of 
the three principal branches (u, v, v), covered with a plexus of sma,ller vessels. The whole 
is dotted over with pigment masses. 

bered that the movements are all magnified, or else one is apt 
to suppose the capillary circulation extremely rapid, whereas 
it is like that of the most sluggish part of a stream, and very 

The Characters of the Blood-Flow. 

If an artery be opened, the blood is seen to flow from it in 
a constant stream, with periodic exaggerations, which, it is 
found, answer to the heart-beats ; in the case of veins and 
capillaries the flow is also constant, but shows none of the 
spurting of the arterial stream, nor has the cardiac beat appar- 
ently an equal modifying effect upon it. 

We have already explained why the flow should be constant, 
though it would be well to be clearer as to the peripheral re- 
sistance. The amount of friction from linings so smooth as 


those of the blood-vessels can not be considerable. Whence, 
then, arises that friction which keeps the arterial vessels always 
distended by its backward influence ? The microscopic study 
of the circulation helps to answer this question. The plas- 
ticity of the corpuscles and of the vessel walls themselves 
must be taken into account, in consequence of which a drag- 
ging influence is exerted whenever the corpuscles touch the 
wall, which must constantly happen with vast numbers of 
them in the smallest vessels and especially in the capillaries. 
The arrangement of capillaries into a mesh-work, must also, in 
consequence of so many angles, be a source of much friction. 

The action of the corpuscles on one another may be com- 
pared to a crowd of people hurrying along a narrow passage — 
the obstruction comes from interaction of a variety of forces, 
owing to the crowd itself rather than the nature of the thor- 
oughfare. We must set down a great deal to the influence of 
the corpuscles on one another, as they are carried along, accord- 
ing to mechanical principles ; but, as we shall see later, other 
and more subtile factors play a part in the capillary circulation. 
Owing to the peripheral resistance and the pumping force of 
the heart, the arteries become distended, so that, during cardiac 
diastole, their recoil, owing to the closure of the semilunar 
valves, forces on the blood in a steady stream. It follows, then, 
that the main force of the heart is spent in distending the 
arteries, and that the immediate propelling force of the circu- 
lation is the elasticity of the arteries in which the heart stores 
up the energy of its systole for the moment. 


Keeping in mind our schematic representation of the circu- 
lation, we should expect that the blood must exercise a certain 
pressure everywhere throughout the vascular system ; that this 
blood-pressure would be highest in the heart itself ; considera- 
ble in the whole arterial system, though gradually diminishing 
toward the capillaries, in which it would be feeble ; lower still 
in the smaller veins ; and at its minimum where the great veins 
enter the heart. Actual experiments confirm the truth of these 
views ; and, as the subject is one of considerable importance, 
we shall direct attention to the methods of estimating and re- 
cording an animal's blood-pressure. 

First of all, the well-known fact that, when an artery is cut, 
the issuing stream spurts a certain distance, as when a water- 


main, fed from an elevated reservoir, bursts, or a hydrant is 
opened, is itself a proof of the existence of blood-pressure, and 
is a crude measure of the amount of the pressure. 

One of the simplest and most impressive ways of demon- 
strating blood-pressure is to connect the carotid, femoral, or 
other large artery of an animal by means of a small glass tube 
(drawn out in a peculiar manner to favor insertion and reten- 
tion by ligature in the vessel), known as a cannula, by rubber 
tubing, with a long glass rod of bore approaching that of the 
artery opened, into which the blood is allowed to flow through 
the above-mentioned connections, while it is maintained in a 
vertical position. 

To prevent the rapid coagulation of the blood in such ex- 
periments, it is customary to fill the cannula and other tubes 
to a certain extent, at least, with a solution of some salt that 
tends to retard. coagulation, such as sodium carbonate or bicar- 
bonate, magnesium sulphate, etc. If other connections are 
made in a similar way with smaller arteries and veins, it may 
be seen that the height of the respective columns, representing 
the blood-pressure, varies in each and in accordance with ex- 

While all the essential facts of blood-pressure and many 
others may be illustrated by the above simple methods, it is 
inadequate when exact measurements are to be made or the 
results to be recorded for permanent preservation ; hence appa- 
ratus of a somewhat elaborate kind has been devised to accom- 
plish these purposes. 

The graphic methods are substantially those already ex- 
plained in connection with the physiology of muscle; but, 
since it is often desirable to maintain blood-pressure experi- 
ments for a considerable time, instead of a single cylinder, a 
series so connected as to provide a practically endless roll of 
paper (Fig. 208) is employed. 

When, in the sort of experiments referred to above, the 
height of the fluid used in the glass tube to prevent coagula- 
tion just suffices to prevent outflow from the artery into the 
connections, we have, of course, in this a measure of the blood- 
pressure ; however, it is convenient in most instances to use 
mercury, contained in a glass tube bent in the form of a U, for 
a measure, as shown in the subjoined illustration. It is also 
desirable, in order to prevent outflow of the blood into the 
apparatus, to get up a pressure in the U-tube or manometer as 
near as may be equal to that of the animal to be employed in 



Fig. SOf.— Apparatus used in making a blood-pressure experiment (after Foster), pb, pressure- 
bottle, elevated so as to raise tne pressure several inches of mercury, as seen in the ma- 
nometer (m) below. It contains a saturated solution of sodium carbonate ; r.f. rubber tube 
connecting the pb with the leaden tube ; l.t, tube made of lead, so as to be pliable, yet have 
rigid walls ; s.c. a stop-cock, the top of which is removable, to allow escape of bubbles of 
air ; p, the pen, writing: on the roll of paper, r. The former floats on the mercury ; m, the 
manometer, the shaded portion of the bent tube denoting the mercury, the rest is filled 
with a fluid unfavorable to the coagulation of the blood, and derived from, the pressure- 



bottle ; ca, the carotid, in which is placed the canula, and below the latter a forceps, which 
may be removed when the blood-pressure is to be actually measured. The registration of 
the height, variation, etc., of blood-pressure, is best made on a continuoiis roll of paper as 
seen in Fig 308. f f , 

the experiment. This may be effected in a variety of ways, 
one of the most convenient of which is by means of a vessel 
containing some saturated sodium carbonate or similar solu- 
tion in connection with the manometer. 

It is important that the pressure should express itself as 
directly and truthfully on the mercury of the manometer as 
possible, hence the employment of a tube with rigid walls, yet 
capable of being bent readily in different directions for the sake 
of convenience. 

Mercury, on account of its inertia, is not free from objec- 
tion ; and when very delicate variations in the blood-pressure — 
e. g., feeble pulse-beats — are to be indicated, it fails to express 
them, in which case other fluids may be employed. 

Fig. 308. — Lar^e kymograph, with continuous roll of paper (Foster). The clock-work ma- 
chinery unrolls the paper from the roll C, carries it smoothly over the cylinder B, and then 
winds it up into the roll A. Two electro-magnetic markers are seen in position recording 
intervals of time on the moving roll of paper. A manometer may be fixed in any con- 
venient position. 

It will be noted that when an ordinary cannula is used, in- 
serted as it is lengthwise into the blood-vessel, the pressure 
recorded is not that on the side of the vessel into which it is 
inserted as when a H - piece is used, but of the vessel, of which 
the one in question is a branch. The blood-pressure, in the 
main arterial trunk for example, must depend largely on the 
force of the heart-beat ; consequently it would be expected, and it 


is actually found, that the pressure varies for different animals, 
size having, of course, in most instances a relation to the result. 
It has heen estimated that in the carotid of the horse the arte- 
rial pressure is 150 to 200 mm. of mercury, of the dog 100 to 175, 
of the rahhit 50 to 90. Man's hlood-pressure is not known, hut 
is prohahly high, we may suppose not less than 150 to 300 mm. 
After the fact that there is a certain considerable blood- 
pressure, the other most important one to notice is that this 
blood-pressure is constantly varying during the experiment, 
and, as we shall give reason to believe, in the normal animal ; 
and to these variations and their causes we shall presently turn 
our attention. 


The heart, being one of the great centers of life, to speak 
figuratively, it demands an unusually close study. 

The Cardiac Movements. 

There is no special difficulty in ascertaining the outlines of 
the heart by means of percussion on either the dead or the 
living subject. Quite otherwise is it with the changes in form 
which accompany cardiac action. Attempts have been made to 
ascertain the alterations in position of the heart with respect 
to other parts, and especially its own alterations in shape dur- 
ing a systole, the chest being unopened, by the use of needles 
thrust into its substance through the thoracic walls ; but the 
results have proved fallacious. Again, casts have been made 
of the heart after death, in a condition of moderate extension, 
prior to rigor mortis ; and also when contracted by a hardening 
fluid. These methods, like all others as yet employed, are open 
to serious objections. 

Following the rapidly beating heart of the mammal with 
the eye produces uncertainty and confusion of mind. We look 
to instantaneous photography to furnish a possible way out of 
the difficulty. 

It may be very confidently said that the mode of contrac- 
tion of the hearts of different groups of vertebrates is variable, 
though it seems highly probable that the divergences for mam- 
mals are slight. The most that can be certainly affirmed of 
the mammalian heart is, that during contraction of the ventri- 
cles they become more conical ; that the long diameter is not 
appreciably altered ; that the antero - posterior diameter is 


lengthened ; and tliat tlie left ventricle at least turns on its own 
axis from left to right. This latter may be distinctly made out 
by the eye in watching the heart in the opened chest. 

The Impulse of the Heaet. 

When one places his hand over the region of the heart in 
man and other mammals, he experiences a sense of pressure 
varying with the part touched, and from moment to moment. 
Instruments constructed to convey this movement to recording 
levers also teach that certain movements of the chest wall cor- 
respond with the propagation of the pulse, and therefore to the 
systole of the heart. It can be recognized, whether the hand 
or an instrument be used, that all parts of the chest wall over 
the heart are not equally raised at the one instant. If the beat- 
ing heart be held in the hand, it will be noticed that during 
systole there is a sudden hardening. The relation of the apex 
to the chest wall is variable for different mammals, and with 
different positions of the body in man. 

As a result of the investigation which this subject has i-e- 
ceived, it may be inferred that the sudden tension of the heart, 
owing to the ventricle contracting over its fluid contents, causes 
in those cases in which during diastole the ventricle lies against 
the chest wall, a sense of pressure beneath the hand, which is 
usually accompanied by a visible movement upward in some 
part of the thoracic wall, and downward in adjacent parts. 
The exact characters of the cardiac impulse are very variable 
with different human subjects. The term " apex-beat " is fre- 
quently employed instead of cardiac impulse, on the assump- 
tion that the apex of the heart is brought into sudden contact 
with the thoracic walls from which it is supposed to recede 
during diastole. But, in some positions of the body at all 
events in a certain proportion of cases, the apex of the heart 
lies against the chest wall during diastole, so that in these 
instances certainly such a view would not be wholly correct. 
But we would not deny that in some subjects there may be a 
genuine knock of the apex against the walls of the chest during 
the ventricular systole. 

It will not be forgotten that the heart lies in a pericardial 
sac, moistened with a small quantity of albuminous fluid ; and 
that by this sac the organ is tethered to the walls of the chest 
by its mediastinal fastenings; so that in receding from the 
chest wall the latter may be drawn after it ; though this might 



also foUo-w from the intercostal muscles being simply unsup- 
ported when the heart recedes. 

Investigation of the Heakt-Beat fkom within. 

By the use of apparatus introduced within the heart of the 
mammal and reporting those changes susceptible of graphic 
record, certain tracings have been obtained about the details of 

Fig. 209.— Marey's cardiac sound which may be used to explore the chambers of the heart 
(after Foster), a. is made of rubber stretched over a wire framework, with metallic 
supports above and below ; b, is a long tube, 

which there are uncertainty and disagreement, though they 
seem to establish the nature of the main features of the cardiac 
beat clearly enough. An interpretation of such tracings in the 

Right auricle. 

Right Tentricle. 

Cardiac impulse. 

Fig. 210. — Simultaneous tracings from the interior of the right auricle, from the interior of the 
right ventricle, and of the cardiac impulse, in the horse (after Chauveau and Marey). 
Tracings to be read from left to right, and the references above are In the order from tOT) 
■ to bottom. A complete cardiac cycle is included between the thick vertical lines I and II. 
The thin vertical lines indicate tenths of a second. The gradual rise of pressure within the 
ventricle (middle tracing) during diastole, the sudden rise with the systole, its maintenance 
with oscillations for an appreciable time, its sudden fall, etc., are all well shown. There ia 
disagreement as to the exact meaning of the minor curves in the larger ones. 

light of our general and special knowledge warrants the fol- 
lowing statement. 


1. Both auricular and ventricular systole are sudden, but 
the latter is of very much greater duration. 

2. While the chest wall feels the ventricular systole, the au- 
riculo- ventricular valves shield the auricle from its shock. 

3. During diastole in both chambers the pressure rises 
gradually from the inflow of blood ; and the auricular contrac- 
tion produces a brief, decided, though but slight rise of press- 
ure in the ventricles. 

4. The onset of the ventricular systole is rapid, its maximum 
pressure suddenly reached, and its duration considerable. 

The relations of these various events, their duration, and the 
corresponding movements of the chest wall; may be learned by 
a study of the above tracing which the student will find worthy 
of his close attention. 

The Cardiac Sounds. 

Two sounds, differing in pitch, duration, and intensity, may 
be heard over the heart, when the chest is opened and the 
heart listened to by means of a stethoscope. These sounds may 
also be heard, and present the same characters when the heart 
is auscultated through the chest wall ; hence the cardiac im- 
pulse can take no essential part in their production. 

The sounds are thought to be fairly well represented, so far 
as the human heart is concerned, by the syllables lub, dup; 
the first sound being longer, louder, lower-pitched, and " boom- 
ing " in quality ; the second short, sharp, and high-pitched. 

In the exposed heart, the first sound is heard most distinct- 
ly over the base of the organ or a little below it ; while the sec- 
ond is communicated most distinctly over the roots of the great 
vessels — that is to say, both sounds are heard best over the 
auriculo-ventricular and semilunar valves respectively. When 
the chest wall intervenes between the heart and the ear, it is 
found that the second sound is usually heard most distinctly 
over the second costal cartilage on the right ; and the first in 
the fifth costal interspace where the heart's impulse is also 
often most distinct. In these situations the arch of the aorta 
in the one case, and the ventricular walls in the other, are close 
to the situations referred to during the cardiac systole ; hence 
it is inferred that, though the sounds do not originate directly 
beneath these spots, they are best propagated to the chest wall 
at these points. 

There are, however, individual differences, owing to a va- 


riety of causes, which it is not always possible to explain fully 
in each case, but owing doubtless in great part to variations it 
the anatomical relations. 

The Causes of the Sounds of the Heart. — There is general agree- 
ment in the view that the second sound is owing to the closure 
of the semilunar valves of the aortic and pulmonary vessels ; 
the former, owing to their greater tension in consequence of the 
higher blood-pressure in the aorta, taking much the larger share 
in the production of the sound, as may be ascertained by listen- 
ing over these vessels in the exposed heart. When these valves 
are hooked back, the second sound disappears, so that there can 
be no doubt that they bear some important relation to the cau- 
sation of the sound. 

In regard to the first sound of the heart the greatest diver- 
sity of opinion has prevailed and still continues to exist. The 
following among other views have been advocated by physi- 
ologists : 

1. The first sound is caused by the tension and vibration of 
the auriculo-ventricular valves. 

2. The first sound is owing to the contractions of the large 
mass of muscle composing the ventricles. 

3. The sound is directly traceable to eddies in the blood. 

In favor of the first view it was argued that by agreement 
the second sound was valvular, and why not the first ? — And 
again that malformations of the valves gave rise to " murmurs" 
(" bruits "), which either obscured or replaced the true sound. 

The second opinion was supported by the fact that the larger 
the heart the more powerful the sound ; that when the blood 
was cut off from the heart by ligature of the vessels success- 
ively, the sound could still be heard ; that with fatty degenera- 
tion of the muscle-fibers of the heart, it had been found that 
the sound was weak — and similar arguments. 

Recently it has been contended very strongly that the first 
sound may be heard by a double stethoscope placed over an ex- 
cised, bloodless, mammalian heart, or even ventricle, while it 
still beats. 

The third opinion was less vigorously upheld, but certain 
experiments and physical phenomena were pointed to in sup- 
port of it. 

Against the arguments adduced above it may be stated that 
the first sound may be conceived as overpowered by a bruit 
without being replaced by it in the proper sense of the word. 
It is well known that the cardiac muscle is peculiar, occupying 



in structure a position intermediate between the striped vol- 
untary fibers and the smooth muscle-cells. Numerous investi- 
gations have shown that the heart is not susceptible of true 

Fio. 311. 

Fio. 312. 

FtG. 211. — Microscopic appearances of fibers from the heart. The cross-striee, divisions 

(brandling), and junctures are visible (Landois). 
Fig. 812.— Muscular flber-cells from the heart. (1 x 425.) a, line of juncture between two 

cells ; b, c, branching cells. 

tetanic contraction, certainly not the heart of the mammal ; so 
that it is customary to term the cardiac contraction peristaltic. 
If this view be correct, how could there be a sound produced by 
muscular contraction alone ? To this it has been replied that 
the sudden tension of the ventricular wall when tightened over 
the blood may give rise to vibrations that account for the 
sound ; and recent investigations have shown that the vibrations 
that give rise to the sound emitted by a contracting skeletal 
muscle may be fewer than was once supposed. The statement 
that a sound may be heard from the excised ventricle under the 
circumstances above mentioned has not been denied; but its 
source has been traced to the action of the heart wall against the 
stethoscope — i. e., some believe the sound to be, in this case, 
of extrinsic origin. Most physicians would be very loath to 
abandon the view that the valves are always to be taken into 
serious account as a factor in the causation of the sound.- 

But, looking at the whole question broadly, is it not unrea- 
sonable to explain the sound resulting from such a complex act 
as the contraction of the heart and what it implies in the light 
of any single factor ? That such narrow and exclusive views 
should have been propagated, even by eminent physiologists, 
should admonish the student to receive with great caution ex- 


planations of the working of complex organs, based on a single 
experiment, observation, or argument of any kind. 

The view we recommend the student to adopt in the light of 
our present knowledge is, that the first sound is the result of 
several causative factors, prominent among which are the sud- , 
d^n tension of the auriculo-ventricular valves, and the contrac- 
tion of the cardiac muscle, not leaving out of the account the 
possible and probable influence of the blood itself through 
eddies or otherwise ; nor would we ridicule the idea that in 
some cases, at all events, the sound may be modified in quality 
and intensity by the shock given to the chest wall during sys-. 

Endo-Caediac Pressures. 

Bearing in mind the relative extent of the, pulmonary and 
systemic portions of the circulation, we should suppose that 
the resistance to be overcome in opening the aortic valves and 
lifting the column of blood that keeps them pressed together, 
would be much greater in the left ventricle than in the right ; 
or, in other words, that the intra-ventricular pressure of the 
left side of the heart would greatly exceed that of the right, 
and this is confirmed by actual experiment. 

By means of an instrument known as the maximuni and 
minimum manometer, the highest and lowest pressure within 
any chamber of the heart may be learned approximately. As 
a specimen measurement it may be stated that it has been 
found that in a dog the greatest pressure was 140 mm. of mer- 
cury for the left ventricle, for the right only 60, and for the 
right auricle 30. But it is also found — a matter not quite so 
obvious — ^that a minimum pressure proportionate to the maxi- 
mum may exist in all the chambers of the heart ; and the press- 
ure may fall below that of the atmosphere, or be negative. By 
the same method it was found that in a dog the negative pressure 
varied between —53 and —30 mm. of mercury for the left ven- 
tricle and —17 to —16 mm. for the right, with —13 to —7 mm. for ' 
the right auricle. As will be shown later, part of this diminished 
pressure is due to the effect of the respiratory movements ; and, 
indeed, more recent experiments seem to show that ordinarily, 
with the heart beating with its usual rate and force, the nega- 
tive pressure or suck from its own action is comparatively 
slight. The discussion of the cause of this negative pressure, 
like the related subject of the cause of the heart's diastole, has 
given rise to much difference of opinion. 



Some find it difficult to understand how the heart after sys- 
tole may regain its original form apart from the assistance of 
diastolic muscles, which are assumed to act so as to antagonize 
those causing systole. 

Others think the elasticity of the heart's muscle sufficient of 
itself to account for the organ's return to its original form. 

But there is surely a misconception involved in both of 
these views. 

If small portions of the heart of the frog, tortoise, or other 
cold-blooded animal, just removed from the body, be observed 
under a microscope it will be seen that they alternately con- 
tract and relax. Now, it is only necessary to suppose that the 
relaxation of the heart is complete after each systole, to under- 
stand how even an empty heart regains its diastolic form. 

That there should be a negative pressure in, say, the left 
ventricle, follows naturally enough from the fact that not only 
are the contents of the ventricle expelled with great sudden- 
ness, but that its walls remain (see Figs. 310 and 314) pressed 
together for a considerable portion of the time occupied by the 
whole systole ; so that in relaxation it follows that there must 

Fio. 213.— Diagram showing the relative heiglit of the blood-pressure in different parts of the 
vascular system (after Yeo). /i, heart ; a, arterioles ; v, small veins ; A^ arteries ; c, cap- 
illaries ; Vy large veins : H, F, representing the zero-line, i, e., atmospheric pressure ; the 
blood-pressure is indicated by the height of the curve. The numbers on the left give the 
pressure, approximately, in mm. of mercury. 

be an empty cavity to fill, or that there must be an aspiratory 
effect toward the ventricle ; hence also one factor in the closure 
of the semilunar valves. 




It thus appears that the heart is not only a force-pump but 
also to some extent a suction-pump ; and, if so, the aspirating 
effect must express itself on the great veins, lacking valves as 
they do, at their entrance into the heart ; hence, with each dias- 
tole the blood would be sucked on into the auricles, a result 
that is intensified by the respiratory movements of the 

Relative Time occupied by the Various Phases of the Cardiac Cycle. 
— The old and valuable diagram reproduced below is meant to 
convey through the eye the relations of the main events in a 
complete beat of the heart or cardiac cycle. The relative 

length of the sounds; the 
long period occupied by the 
pause ; the duration of the 
ventricular systole, which 
it is to be observed is in 
excess of that of the first 
sound, are among the chief 
facts to be noted. 

The tracings of Chau- 
veau and Marey, obtained 
from the heart of the horsoj 
which has a very slow 
rhythm, show that of the 
whole period, the auricular 
systole occupies -J- or y% of 
a second ; the ventricular 
systole, f or ^ of a sec- 
ond ; and the diastole, -f or ^^ of a second. 

With the more rapid beat in man (70 to 80 per minute), the 
duration of the cardiac cycle may be estimated at about ^^ of 
a second, and the probable proportions for each event are about 
these: The auricular systole, -^ oi a, second; the ventricular 
systole, 3^ of a second ; and the pause, 3^ of a second. 

It will be noted that the pause of the heart is equal in dura- 
tion to the other events put together ; and even assuming that 
there is some expenditure of energy in the return (relaxation) 
of the heart to its passive form, there still remains a consider- 
able interval for rest, so that this organ, the very type of cease- 
less activity, has its periods of complete repose. 

Fig. 214. — Diagram representing the movements 
and sounds of tiie heart during a cardiac cycle 
(after Sharpey). 


The "Woek of the Heart. 

Since the pressure against wMcli the heart works must, as 
■we shall see, vary from moment to moment, and sometimes very 
considerably, the work of the heart must also vary within wide 
limits, even making allowance for large adaptability to the bur- 
den to be lifted ; for it will be borne in mind that the degree 
to which the heart empties its chambers is also variable. 

If one knew the quantity of blood ejected by the left ven- 
tricle, and the rate of the beat, the calculation of the work 
done would be an easy matter, since the former multiplied by 
the latter would represent, as in the case of a skeletal muscle, 
the work of the muscles of the left ventricle ; from which the 
work of the other chambers might be approximately calculated. 

The work of the auricles must be slight, considering that the 
filling of the ventricles is not dependent solely upon their con- 
traction, that they empty themselves very imperfectly, and 
that the tracing on Marey's curves (Fig. 210), representing the 
effect of their contraction on the intraventricular pressure is 
but small. Notwithstanding, as they largely determine by 
their contraction and the quantity they throw into the ventri- 
cles how full the latter shall be in a given instance, they really 
have a very large share in determining the total work of the 
ventricles and the whole heart. 

The right ventricle, it is estimated does from one fourth to 
one third the work of the left ; not, of course, because it throws 
out less blood, for if this were the case the left side of the heart 
must soon become empty, not to mention other disturbances of 
the vascular equilibrium, but because of the relatively less 
resistance offered by the pulmonary vessels. 

All attempts to estimate exactly the quantity ejected by the 
left ventricle seem to show that this varies very greatly, after 
due allowance is made for the imperfection of the methods and 
the great discrepancies in the results of different observers. 
Perhaps six ounces, or about 180 grammes, may be taken as an 
average for the left ventricle of man. Assuming that his aortic 
blood-pressure is, say 200 mm. of mercury or 3'21 metres of 
blood, the work of this chamber for each beat would be 180 X 
3"21, or 578 gramme-metres. If the heart beats seventy times per 
minute, the work for the day would be 578 X 70 X 60 X 24 = 58,- 
262,400 gramme-metres. Or, upon the same basis, and assuming 
that the blood makes up about the one thirteenth of the weight 
of the individual, in a man of 143 pounds, the whole of the 



blood would pass through the heart in about thirty beats, or 
in less than half a minute. 

When we calculate the work done by the heart for certain 
intervals, as the day, the week, month, year, and especially for 
a moderate lifetime, and compare this with that of any ma- 
chine it is within the highest modern skill to construct, the 
great superiority of the vital pump in endurance and worldng 
capacity will be very apparent ; not to take into the account at 
all its wonderful adaptations to the countless vicissitudes of life, 
without which it would be absolutely useless, even destructive 
to the organism. 

Some of these variations in the working of the heart we may 
now to advantage consider. 

Variations in the Cardiac Pulsation. 

These may be ascertained either by the investigation of the 
arteries or of the heart, for every considerable alteration in the 
working of the heart expresses itself also through the arterial 
system. In speaking of the pulse, the reference is principally 
to the arteries, but in each case we may equally well think of 
the heart primarily as acting upon the arteries. 

1. ThB frequency of the heart-beat varies, as might be sup- 
posed, with a great multitude of conditions, the principal of 
which are : age, being most frequent at birth, when it may be 
140 per minute, gradiially slowing to old age, when it may fall 
to 60. In feeble old age the heart-beat may, like many other of 
the functions of the body, approximate the infantile condition, 
being very frequent, small, feeble, and easily disturbed in its 

It is a matter of no small importance to the medical student 
to be aware of the normal rate for different periods of life, 
hence we give below a pretty full statement of the variations 
with age. It will be understood that the numbers are only ap- 
proximative, and that large allowance must be made for indi- 
vidual deviations : 
At birth, 130-140 At 4 years, 96-94 At 20 years, 78-72 

1 year, 120-130 " 5 " 94^90 30 " 75-70 

3 years, 100-110 10 " 90-85 50 " 70-65 

3 " 100- 96 15 " 80-75 

Sex. — The cardiac beat is more frequent in females ; stature, 
more frequent in the short ; postwre, most rapid in the standing 
position, slower when sitting, and slowest in the recumbent 


posture ; season, more frequent in summer ; period of the day, 
more frequent in the. afternoon and evening ; elevation of tem- 
peratv/refthe inspiratory act, emotions and mental activity, eating, 
muscviar exercise, etc., render tlie heart-beats more frequent. 

2. The length of the systole, though, variable, is more con- 
stant than that of the diastole. The estimated limits of the 
systole may be stated as "327 to "301 second. 

3. The force of the pulsation varies very greatly and exer- 
cises an important influence on the blood-pressure, and the 
velocity of the blood-stream. As a rule, when the heart beats 
rapidly, especially for any considerable length of time, the force 
of the individual pulsations is diminished. 

4. The heart-beat may vary much and in ways it is quite 
possible to estimate, both directly by the hand placed over the 
organ on the chest, by the modifications of the cardiac sounds, 
and by_ the use of instruments. It is wonderful how much in- 
formation may be conveyed, without the employment of any 
instruments, through palpation and auscultation, to one who 
has long investigated the heart and the arteries with an intelli- 
gent, inquiring mind ; and we strongly recommend the student 
to commence personal observations early and to maintain them, 

Physicians recognize the pulse (and heart) as " slow " as dis- 
tinguished from " infrequent," " slapping," " heaving," " thrill- 
ing," " bounding," etc. 

Now, if with these terms there arise in the mind correspond- 
ing mental pictures of the action of the heart under the cir- 
cumstances, well ; if not, there is a very undesirable blank. 
How the student may be helped to a knowledge of the actual 
behavior of 'the heart under a variety of conditions we shall 
endeavor to explain later. 

Apart from all the above peculiarities, the heart may cease 
its action at regular intervals, or at intervals which seem to 
possess no definite relations to. each o,ther — that is, the heart 
may be irregular in its action, which may be made evident 
either to the hand or the ear. 

There are certain deviations from the quicker rhythm which 
occur with such regularity and are so dependent on events that 
takes place in other parts of the body that they may be con- 
sidered normal. Reference will shortly be made to these and 
the causes of the variations enumerated in this section. 

Comparative. — The following table gives the mean number of 
cardiac pulsations per minute (after Gamgee) : 





Ass and mule. . 


Sheep and goat 





36- 40 
46- 50 
45- 50 
70- 80 
70- 80 


60- 73 

65- 75 

60- 70 

85- 95 




Old age. 

32- 33 
55- 60 
40- 45 
55- 60 
55- 60 
60- 70 

The variations with age, for the horse and the ox, are as fol- 
lows, according to Kreutzer : 


At birth 100-120 

When 14 days old 80-96 

When 3 months old 68-76 

When 6 months old 64-73 

When 1 year old 48-56 

When 3 years old 40-48 

When 3 years old 88-48 

When 4 years old 38-50 

When aged 33-40 


At birth 92-133 

When 4-5 days old 100-130 

When 14 days old 68 

When 4-6 weeks old 64 

When 6-12 months old 56-68 

For the young cow 46 

For the four-year-old ox 40 

Thb Pulse. 

Naturally the intermittent action of the heart gives rise to 
corresponding phenomena in thei elastic tubes into which it 
may be said to be continued, for it is very desirable to keep in 
mind the complete continuity of the vascular system. 

The following phenomena are easy of observation : When a 
finger-tip is laid on any artery, an interrupted pressure is felt ; 
if the vessel be laid bare (or observed in an old man), it may 
be seen to be moved in its bed forward and upward ; the press- 
ure is less the farther the artery from the heart ; if the vessel 
be opened, blood flows from it continuously, but in spurts ; if 
one finger be laid on the carotid and another on a distant ves- 
sel, as one of the arteries of the foot, it may be observed (though 
it is not easy, from difficulty in attending to two events. hap- 
pening so very close together) that the beat in the nearer ves- 
sel precedes by a slight interval that in the more distant. 

Investigating the latter phenomenon with instruments, it is 
found that an appreciable interval, depending on the distance 
apart of the points observed, intervenes. 

What is the explanation of these facts ? 

The student may get at this by a few additional observa- 
tions that can be easily made. 



If water be sent through a long elastic tube (so coiled that 
points near and remote may be felt at the same time) by a bulb 
syringe, imitating the heart, and against a resistance made by 
drawing out a glass tube to a fine point and inserting it into 
the terminal end of the rubber tube, an intermittent pressure 
like that occurring in the artery may be observed ; and further 

Fig. 315.— Marey's apparatus for showine the mode in which the jnilse is propagated in the 
arteries. 5, a rubber pump, with valves to prevent regurgitation. The working of the 
apparatus will be apparent from the inspection of the figure. 

that it does not occur at precisely the same moment at the two 
points tested. 

Information more exact, though possibly open to error, may 
be obtained by the use of more elaborate apparatus, and the 
graphic method. 

Fig. 216 gives an idea of the main features of the pulse-trac- 
ings of an arterial scheme or arrangement of tubes in supposed 
imitation of the conditions existing in the vascular system of 
the mammalian body. Attention is especially directed to the 
abrupt ascent, the more gradual descent, and the secondary 
waves, which are either waves of oscillation or reflex waves. 

It may also be noticed that the rise is later as the part of 
the tube at which it occurs is more distant from the pump ; 
also that it gets gradually less in height and at the same time 
that all the secondary waves are diminished or totally disap- 
pear ; and with the exception of the latter these results hold 
good of the pulse in the arteries of a living animal. 

By measurement it has been ascertained that in man the 
pulse-wave travels at the rate of from five to ten metres per sec- 
ond, being of course very variable in velocity. It would seem 
that the more rigid the arteries the more rapid the rate, for in 



children with their more elastic arteries the speed is slower ; 
and the same principle is supposed to explain the higher veloci- 



50 V\ 

Fig. 316.— Pulse-our»-es described by a series of sphygmogra/phic levers placed 20 cm. apart 
along an elastic tube into which fluid is forced by the sudden stroke of a pump. The 
arrows indicate the onward and the reflected waves. The gradual flattening and total or 
partial extinction of the waves are noteworthy (after Marey). 

ty noticed in the arteries of the lower extremities. But with 
such a speed as even five metres a second it is evident that with 
a systole of moderate duration (say "3 second) the most distant 
arteriole will have been reached by the pulse- wave before that 
systole is completed. 

It is known that the blood-current at its swiftest, has no 
such speed as this, never perhaps exceeding in man half a metre 
per second, so that the pulse and the blood-current must be two 
totally distinct things. 



The student may very simply illustrate this matter for him- 
self. By tapping sharply against a pipe through which a 
stream is flowing slowly and quietly, a wave may be seen to 
arise and pass with considerable velocity along the moving 
water, and with a speed far in excess of the rapidity of the main 
current. When the left ventricle throws its six ounces of blood 
into vessels already full to distention, there must be consider- 
able concussion in consequence of the rapid and forcible nature 
of the cardiac systole, and this gives rise to a wave in the blood 
which, as it passes along its surface, causes each part of every 
artery in succession to respond by an elevation above the gen- 
eral level, and it is this which the finger feels when laid upon 
an artery. 

That there is considerable distention of the arterial system 
with each pulse may be realized in various ways, as by watch- 
ing and feeling an artery laid bare in its course, or in very 
thin or very old people, and by noticing the jerking of one leg 
crossed over the other, by which method in fact the pulse-rate 
may be ascertained. And that not only the whole body but 
the entire room in which a person sits is thrown into vibration 
by the heart's beat, may be learned by the use of a telescope to 
observe objects in the room, which may thus be seen to be in 

Features of an Arterial Pulse-Tracing. — In order to judge of the 
nature of arterial tracings, it is important that the circum- 
stances under which they are obtained should be known. 

The movements of the vessel wall in most mammals suit- 

FiG. 817.— Marey's improved sphygmograph arranged for taking a tracing. A, steel spring ; 
B, first lever ; C, writing-lever ; C, its free writing end ; D, screw for bringing B m con- 
tact with C ; G, slide with smoked paper ; H, clock-work ; Z,, screw for moreasing the 
pressm'e ; M, dial indicating the amount of pressm:e ; K, K, straps for fixing the instru- 
ment to the arm, and the latter to the double-inclined plane or support (Byrom Bramwell). 



Fig. 218.— Diagrammatic schema sliowing 
the essential part of the instrument 
when in woricine order. The knife-edge, 
B"^ of the short lever is in contact with 
the writing-lever, G. Every movement 
of the steel spring at A'\ communica- 
ted by the arteries, will be imparted to 
the writing-lever (Byrom Bramwell). 

able for experiment and in man is so slight that it becomes ne- 
cessary to exaggerate them in the tracing, hence long levers are 
used to accomplish this. 

The sphygmograph is the usual form of instrument em- 
ployed for the purpose. It consists, essentially, of a clock-work 
for moving a smoked surface (mica plate commonly) on which 

the movements of a lever-tip, 
answering to those of a button 
placed on the artery, are re- 

Considering the nature of the 
pulse and the apparatus em- 
ployed to wfite its characters, it 
will be seen that the possible 
sources of error are numerous. 

Different observers have, as 
a matter of fact, even with the 
same sort of instrument obtained tracings differing not a little 
in character. As the subjoined figures show, the pressure ex- 
erted upon a vessel may so alter the result that entire features 
of the tracing may actually disappear. The sphygmograph, 
even in the most skillful hands, has proved somewhat disap- 
pointing as a physiological and especially as a clinical instru- 
ment, though it is not without a certain value. 

We shall do well to inquire whether there are any features 
in common in tracings obtained in various ways, and which 
have therefore in all probability a real foundation in nature. 
An inspection of a 

ixr^e^s: fwrnmrnrnx 

diverse conditions, 
seems to show that in 
all of them there oc- 
curs, more or less 
marked, the follow- 
ing : 1. An upward 
curve. 3. A downward 
curve, rendered irreg- 
ular by the occurrence 
of peaks or crests and 
notches. The first of these are termed the predicrotic notch and 
crest, and the succeeding ones the dicrotic notch and crest. 
The latter seem to be the more constant. 

Fig. 319.— I*ulse-traeing from carotid artery of healthy 
man (after Moens). x, commencement of expansion 
of artery ; A, summit of first rise ; C, dicrotic second- 
ary wave ; B, predicrotic secondary wave ; p, notch 
preceding this ; Z), succeeding secondary wave. Curve 
above is that made by a tuning-fork with ten double 
vibrations in a second. 



Tliat these are genuine, answer of real and corresponding 
elevations of tlie arterial wall and of the blood-current itself, 
seems probable from 
the study of a Jicemau- 
tograTn. The latter may 
be obtained by allowing 
the blood from a cut 
artery to spurt against 
a piece of paper drawn 
in front of the blood- 
stream. It is also as- 
serted that by a tele- 
phonic connection with 
an artery both the pri- 
mary pulse-wave and 
the dicrotic wave may 
be heard. More rarely 
there are interruptions 
in the first upward 

curve, termed anacrotic curves, as distinguished from those in 
the downward curve known as katacrotic. 

It has been generally admitted that the first marked upward 

curve is due to the systolic 


The following are, in 

brief, some of the views 

that have been entertained 
in regard to the minor features of the tracings : 

(a.) That the predicrotic wave-crest is owing to the sudden 
arrest of the flow from the ventricle. 

(6.) That the dicrotic wave is a wave of oscillation. 

Fig. 320. — Piilse-curve from radial of man. Taken with 
an extra-Ta£cular pressm-e of 70 mm. of mercury. 
The (nirved interrupted lines show the distance from 
one another in time of the chief phases of the pulse- 
wave, a;, the commencement, and A, the close of ex- 
pansion of artery ; p, predicrotic notch ; d, dicrotic 
notch ; C, dicrotic crest ; D, post-dicrotic crest ; /, 
the post-dicrotic notch. 

Fig. 281.- 

-Anacrptic pulse-tracing from carotid of 

Fig. 238.— Two grades of marked dicrotism in radial pulse of man (typhoid fever). 

(c.) That it is a wave of reflection from the periphery. 

(cZ.) That it is caused by the sudden closure of the aortic 

It appears to be now pretty well agreed that the theory of 
reflection is untenable on physical principles ; that a high 
blood-pressure tends to render the katacrotic markings less 



distinct, and the reverse "when the pressure is low, as after 
hsemorrhages. These features are especially marked in the 


Fio. 223.— Normal pulse-ourve in the aorta from the dog. 

dicrotic pulse of fever, etc., when the blood-pressure is low and 
may be recognized even by the hand. The anacrotic crests 
and notches are abnormal, and probably 
due to excessive rigidity of the arteries. 

Certain it is that, without any change 
in the heart-beat, changes in the tracings 
may arise, owing to modifications in the 
periphery of the vascular system. We do 
not propose to discuss the above-men- 
tioned views of the causation of the minor 
features of the tracings in detail, about 
which the greatest differences of opinion 
if all the characteristics of an arterial 

Fio. 224.— Anacrotic sphyg- 
mograph tracing from 
the ascending aorta in a 
case of aneruism. 

still prevail. Even 
tracing could be ob- 
tained from an arti- 
ficial schema, it 
would not follow 
that the conditions 
in each case were 
the same ; in fact, 
as we view the mat- 
ter, it would be all 
but impossible that 
such should be the 

Rubber tubes are 
not comparable to 
arteries ; and espe- 
cially not to arteri- 
oles and capillaries. 
Bearing in mind the 
peculiar nature of the blood-corpuscles; their relation to the 
walls of the vessels in which they flow; the relation of the 

Fig. 235.— Influence of changes in the pressure applied to 
the exterior of the vessel (extra-vascular) on the form of 
the curve, a, from the radial of a man of twenty-seven 
years, with an extra-arterial pressure of, in a, 70 mm., 
in a', to 50 mm., and in a", to 30 mm. mercury. 


blood to the mitritipn of the tissues ; the fact that all the tubes 

that compose the vascular system are made up of living cells ; 

that some of these 

cells (in arterioles and 

capillaries) are in a 

semi-fluid condition — 

in a word, that the 

conditions of the cir- I^WK/SA/n^ 
culation as a whole " 

T_ FiQ. 326.— Dicrotic pulse-curve due to haemorrhage. From 

are Sm genenS, be- carotid ot rabbit, with extra-vasoular pressure of , in a, 

f . -1 • 'i. ■] 'J. 50 mm., 6, of 40 mm., c, of 20 mm., and d, of 10 mm. 

cause 01 Dneir Vluailuy mercury. (TIlis and the preceding six tracings from 

— it seems to us amaz- I'oster.) 

ing that purely physical explanations, such as would answer 
for a pump and set of rubber tubes, should ever have been 
deemed satisfactory. The whole subject seems to be involved 
in a gross misconception, and should be regarded, we must 
think, from an entirely new standpoint. 

Venous Pulse. — Apart from the variations in the caliber of 
the great veins near the heart, constituting a sort of pulse, 
though due to variations in intra-cardiac pressure, a venous 
pulse proper is rare as a normal feature. One of the best- 
known examples of such occurs in the salivary gland. When, 
during secretion, the arterioles are greatly dilated, a pulse may 
be witnessed in the veins into which the capillaries open out, 
owing to diminution in the resistance which usually is suffi- 
ciently great to obliterate the pulse-wave. 

Pathological. — In severe cases of heart-disease, owing to 
cardiac dilatation or other conditions, giving rise to incompe- 
tency of the tricuspid valves, there may be with each ventricu- 
lar systole a back-flow, visible in the veins of the neck. 

A venous pulse is a phenomenon, it will be evident, that 
always demands special investigation. It means that the usual 
bounds of nature are for some good reason being over-stepped. 

Comparative. — Before entering on the consideration of phe- 
nomena that all are agreed are purely vital, we call attention to 
the circulation in forms lower than the mammal, in order to 
give breadth to the student's views and prepare him for the 
special investigations, which must be referred to in subsequent 
chapters ; and which, owing to the previous narrow limits (re- 
searches upon the frog and a few well-known mammals) having 
at last been overleaped, have opened up entirely new aspects of 
cardiac physiology — one might almost say revolutionized the 


Owing to the limitations of our space, the references to lower 
forms must be brief. 

We recommend the student, however, to push the subject 
further, and especially to carry out some of the experiments to 
which attention will be directed very shortly. 

In the lowest organisms (Infusorians) represented by Amoe- 
ba, Vorticella, etc., there are, of course, no circulatory organs, 
unless the pulsating vacuoles of some forms mark the crude 
beginnings of a heart. It will be borne in mind, however, that 
there is a constant streaming of the protoplasm itself within 
the organism. 

Among Ccslenterates (Figs. 254, 355) the digestive system, as 
yet but imperfectly developed, seems to embody in itself a sort 
of combination of the functions of the preparation and distribu- 
tion of elaborated food ; and it is worth while to note that even 
in the highest animals the digestive tract remains in close con- 
nection with the circulatory system. 

The heart is first represented, as in worms, by a pulsatile 
tube, which may, as in the earth-worm, extend throughout the 
greater part of the length of the animal, and has usually dorsal 
and ventral and transverse connections. 

The dilatations of the transverse portions in one division 
{metamere) of the animal seem to foreshadow the appearance of 

The pulsation of the dorsal vessel in a large earth-worm is 
easy of observation. 

In the moUusks the heart consists of a ventricle and one or 
more auricles, and these chambers give off and receive large 
vessels (Fig. 237). 

These hearts may be observed pulsating with the naked eye 
or a lens in the clam, oyster, or snail, and are to be looked for 
in the first two on the side of the animal toward the hinge of 
the shell. 

It is worthy of note that in cephalopod moUusks (Cuttle- 
fish, Poulpe) there are branchial hearts, which may be re- 
garded in the light of pulsatile venous expansions, a remnant, 
perhaps, of conditions found in lower forms, in which we have 
seen that the rhythmically contracting tube plays a prominent 

In amphioxus, which is often instanced as the lowest verte- 
brate, the blood-vessels, including the portal vein, are pulsatile, 
while there is no distinct and separate heart ; but, in connection 
with the above observations in cephalopods, it is to be re- 



marked that in this creature there are contractile dilatations at 
the bases of the branchial arteries. 

Fio. 227.— Circulatory and excretory organs of the cuttle-flsh (Sepia officinalis), viewed from 
the dorsal side (after Hunter). Br, gills ; C, ventricle ; Ao and Ao', anterior and poste- 
rior aorta ; F, lateral vein ; Vc\ anterior vena cava ; Fc", posterior vena cava : N, renal 
appendages of the veins ; Vlr^ advehent branchial vessels (branchial arteries) ; JT/i, 
branchial heart ; Ap, appendsige of the same ; At, At', auricles receiving the revehent 
branchial vessels (branchial veins). 

In some Ascidians the heart is of a somewhat crescentic 
form, and has the remarkable property of beating for a time in 
one direction, then stopping and reversing its rhythm. In a 
transparent specimen, under the microscope, this can be seen 

In the crab the heart lies within a pericardium, loosely at- 
tached, the main vessels being connected with the pericardium 
and not directly with the heart. The heart sucks its blood 
from the pericardial cavity through four valvular openings. 

In such a creature as the scorpion there is a chambered 
heart, with a division for each principal segment of the animal's 
body (Fig. 308). 

While in mollusks, crustaceans, and other groups, the vas- 
cular system does not form a connected whole, the scorpion is 
exceptionally advanced in this respect, being provided with 
capillaries, or tubes closely representing them. Among most 
of the invertebrates the blood, after leaving the arteries, passes 
into rather wide, irregular spaces among the various tissues, 
from which it is taken up by the veins without the intervention 
of an intermediate set of vessels. 

The circulatory system of an insect or crustacean may be 



viewed microseopically in aquatic forms, which are often quite 
transparent, especially in the larval condition. 

— ffi S Sjs 
> Sol's fc 

S BO ..dirt 

i'c.s'a . 

V <u [o g CO 


o s F»r *^ a> 5 

' Piu £ 

Although the respiratory system will be treated from the 
comparative point of view, the student will do well to note now 


Ab Ao K 


Fio. 229.— Diagram of the circulation of a Teleoatean fish (Claus). F, ventricle ; Ba, bulbus 
arteriosus, with the arterial arches which carry the blood to the ^ills ; Ab. arterial arches ; 
Ao, aorta descendens, into which the epibranchial arteries passing out from the gills 
unite ; K, kidneys ; /, intestine ; Pc, portal circulation. 

(in the figures) the close relation between the organs for dis- 
tributing and aerating the blood. 

Fio. 330. 

Fio. 231. 

Fio. 830.— The arterial trunks and their main branches in the frog {Bona esculenta). 1 x IJ. 
(Howes.) I. lingual vessel ; c. c, common-carotid artery ; p. cu, pulmo-cutaneous artery ; 
c. gl, carotid gland ; ou', right auricle ; au", left auricle ; v, ventricle ; tr. a, truncus ar- 
teriosus ; pvV, pulmonary ; Ig", left lung ; ao' , left aortic arch ; br, brachial ; cu, cuta- 
neous ; d. ao, dorsal aorta ; coe, cceliaco-mesenteric ; cce\ coeliac ; np, hepatic vessels ; 
g, gastric ; pc', pancreas ; m, mesenteric ; sp, splenic ; du', duodenal ; h, hsemorrhoidal ; 
W, ileal ; hy, hypogastric ; c. z7. common-iliac ; re, renal ; Ic, kidney ; is, spermatic. 

Fio. ]al.— Venous trunks and their main branches in the frog (Bana esculenta). 1 x IJ. 
(Howes.) !, lingual vein ; ' ■ ■• ■ • • .... ... 

s. ac, subscapula 
lobe of liver ; Iv , 

lumbar ; od, oviducal ; r. b, renal-portal ; fm, femoral ; sc, sciatic ; a, femoro-sciatic 
anastomosis ; pv', right pelvic ; vs, vesical ; ant. ab, anterior abdominal ; a', abdominal- 
portal anastomosis ; il\ ileal ; sp, splenic ', du', duodenal ; I. int, lieno-intestinal ; g, gas- 
tric ; p, portal ; Ig", left lung ; pvX, pulmonary ; m. cu, musculo-cutaneous ; br, brachial. 



Passing on to the vertebrates, in the lowest group, the fishes, 
the heart consists of two chambers, an auricle and a ventricle, 
the latter being supplemented by an extension {bulbus arterio- 
sus) pulsatile in certain species; and an examination of the 
course of the circulation will show that the heart is through- 
out venous, the blood being oxidized in the gills after leaving 
the former. 

Among .the amphibians, represented by the frog, there are 
two auricles separated by an almost complete septum, and one 


FlQ. 232. 

Fie 233. 

Fio. 232.— The frog's heart, seen from the front, the aortic arches of the left side having been 
removed. (1 x 4.) ca^ carotid ; c. gl, carotid gland ; ao^ aorta ; au\ right auricle ; au", 
left auricle ; pr. c, vena cava superior ; pt. c. vena cava inferior ; p. e«, pulmo-cutaneous 
trunk ; <r, truncus arteriosus ; u, ventricle (Howes). 

Fig. ^. — The same, seen from behind, the sinus venosus having been opened up to show the 
sinu-auricular valves. (1 x 4.) p. u pulmonary vein ; s. v, sinus venosus ; va'\ sinu-au- 
ricular valve. Other lettering as in Fig. 232 (Howes). 

ventricle characterized by a spongy arrangement of the mus- 
cle-fibers of its walls. 

In the reptiles the division between the auricles is complete, 
and there is one ventricle which shows imperfect subdivisions. 

In the crocodile, however, the heart consists of four per- 
fectly divided chambers. Of the two aortic arches, one arises 
together with the pulmonary artery from the right ventricle, 
and, as it crosses over, the left communicates with it by a small 
opening, so that, although the arterial and the venous blood 
are completely separated in the heart, they intermingle outside 
of this organ. 

In birds the circulatory system is substantially the same as 
in mammals ; but in all vertebrate forms below birds the blood 
distributed to the tissues is imperfectly oxidized or is partially 



As an example of the influence of valves and of blood-press- 
ure on the distribution of the blood we may take the case of the 
turtle, in which the subject has been most carefully studied. 

FlQ. 834. 

Fig. 835. 

Fig. 234.— The heart, dissected from the front, the ventral wall and one of the auriculo-ven- 
tricular valves having been removed. (1 x 6.) The rod, passing from the ventricle into 
the pylanglum, shows the course taken by the blood flowing into the carotid and aortic 
trunks, sy, sjnangium ; p. v', aperture of entry of pulmonary vein ; va'\ sinu-auricular 
valve ; a. s, inter-auricular septum ; va'. auriculo-ventricular valve ; va. s, semi-lunar 
valves ; py^ pylangium ; va, I, longitudinal valve (septum) of pylangium ; p. cu\ point of 
origin of pulmo-cutaneous trunk (Howes.) 

Fig. 335. — Heart and arteries of a reptile (boa), r, right, and Z, left auricle ; c, carotid arteiy; 
ro, right aortic arch : la, left aortic arch ; p, pulmonary artery ; ru, right vena cava ; Iv, 
left vena cava superior ; vi, vena cava inferior. The arrows indicate the course of the 
circulation (after Gegenbaur). 

The structure of the heart and the relations of its main ves- 
sels, etc., will probably be sufficiently clear upon an examina- 
tion of the accompanying figures and the descriptions beneath 

The right and left auricles pour their blood, kept somewhat 
apart by valves, into the cavum venosum. 

Two arterial arches arise from the right-hand part of this 
region, while the pulmonary artery is a branch carrying off 
blood to the lungs from the cavum pulmone. No vessels arise 
from the cavum arteriosum. 

Since the blood flows in the direction of least resistance 
when the ventricle contracts, the venous blood of the cavum 
venosum passes on into the pulmonary artery in which the 
pressure is, of course, lower than in the aortic arches, but, as 
the systole continues, the arterial blood of the cavum arterio- 



sum crowds on the venous blood and passes itself with some of 
the darker blood into the aortic vessels, in which the arrange- 


Fig. 336. 

Fig. 837. 

Fig. 236.— Heart and arteries of a turtle (Chelydra). rp, right pulmonary, and Ip, left pul- 
monary artery ; other letters the same signification as in the previous figure (after Gegen- 

Fig. 337.— Heart of a turtle (.Chelone midas). A. Drawing from nature, the ventral face of 
the ventricle being laid open. B. Diagram explanatory of the circulation. Everywhere 
the arrows indicate the course the blood takes. R.A.^ L.A., right and left auricles, 
•y, the right, v', the left median auriculo-ventricular valves. C. v, cavum venoswm. C. p, 
cavum pulmonale, a, the incomi>lete s^tum which divides the cavum pulmonale from 
the rest of the cavity of the ventricle. P. A, pulmonary ai'tery. B. Ao, L. Ao, right and 
left aortse (after Huxley). 

ment of the valves assists materially. Note that, as the systole 
advances, the imperfect septum between the cavum pulmonum 
and cavum venosum approaches the back of the heart wall, and 
thus tends to shut oflE the cavum pulmone from the purer blood. 

As a result of the entire arrangement, the least oxidized 
blood passes to the lungs, and the most aerated to the head and 
anterior parts of the animal. 

In the frog and other creatures, with three imperfectly sepa- 
rated heart cavities, a similar result is attained. 

The resemblances in such cases to the foetal conditions in 
mammals, including man, will be apparent, and it is especially 


to be observed that in the case of the fcetus and these lower 
groups of vertebrates the brain and anterior parts — that is, 
the most important portions of the animal functionally, 
the parts on which the rest depend for their well-being (since 
the brain is the seat of all the main directive centers) — are 
fed with the best blood the organism possesses, a fact which 
probably explains in part the relatively large size of these 
portions of the body early in foetal life and throughout its 

We now urge upon the student the importance of making 
some observations for himself upon the heart of the frog, tur- 
tle, snake, fish, or other of the cold-blooded animals. At- 
tention should be given chiefly to the functions of the heart, 
though to do this intelligently it must be preceded by some 
study of the anatomy of the organ. It will be understood that 
any directions we may give for the manipulative part of the 
work will be of the simplest kind, and rather suggestive of the 
general method of procedure than intended to illustrate the 
best methods. 

In reality, it is better for exact investigation of the heart 
that no anaesthetic be given, and an animal may be rendered 
insensible by a sudden blow upon the head, which, as we shall 
show later, may be painless. However, it will be, upon the 
whole, perhaps, best that the animal be given a few whiffs of 
ether beneath some (glass) vessel, and as soon as it becomes 
insensible, to withdraw the anaesthetic, remove or crush the 
head (brain), so that throughout the investigation there may 
be neither interference with the heart from this organ nor any 
doubt about the animal's insensibility. 

It is well to open the abdomen a little below the heart, so 
that the latter may be exposed, with its pericardium intact, 
when the relations of the heart to the surrounding parts may 
be noticed. 

What strikes every observer is the sluggish action of the 
hearts of these animals — a great advantage in attempting to 
estimate roughly the relative time occupied by the systole and 
diastole of the different chambers ; the peculiar vermiform 
nature of the contraction ; the changes of color dependent on 
the degree to which any chamber is filled with blood ; and 
many of those minor details important in making up a total 
general impression, but not readily expressed in words. 

After the animal has been bled, the heart's action may still 
be profitably studied ; and, finally, it may be learned that the 


heart will pulsate when removed, either entire or after being 
divided into sections. 

.In another specimen it would be desirable to allow the 
heart, to be kept bathed in serum or physiological saline solu- 
tion, to beat as long as it will, and to note the various phases 
of irregularity, weakening, and cessation of action in its dif- 
ferent parts. 

It is also highly instructive to observe the effect of ligating 
off certain of the chambers from the rest of the organ. 

Any one who makes a few such observations will be pre- 
pared to comprehend readily any of the experiments on the 
hearts of the cold-blooded animals, and will be able, especially 
if he has followed out earlier recommendations as to the study 
of the heart of the mammal, to form a mental picture of what 
is transpiring within his own breast, which is one of the most 
desirable accomplishments — in fact, the best test of real knowl- 

Whatever ground for differences of opinion there may be 
as to the extent to which the phenomena we have as yet been 
describing are mechanical in their nature, all are agreed that 
such explanations are insufficient when applied to the facts 
with which we have yet to deal. They, at all events, can be 
regarded only as the result of vitality. 

When one reflects upon the vicissitudes through which an 
animal must pass daily and hourly, necessitating either that 
they be met by modified action of the organs of the body or 
that the destruction of the organism ensue, it becomes clear 
that the varying nutritive needs of each part must be met by 
changes in the circulatory system. These changes may affect 
any part of the entire arrangement, and it rarely happens, as 
will appear, that one part is modified without a correspond- 
ing one, very frequently of a different kind, taking place in 
some other. What these various correlated modifications are, 
and how they are brought about, we shall now attempt to 
describe, and it will greatly assist in the comprehension of the 
whole if the student will endeavor to keep a clear mental pict- 
ure of the parts before his mind throughoiit, using the figures 
and verbal descriptions only to assist in the construction of 
such a mental image. We shall begin with the vital pump — 
the heart. 


The Beat of the Heart and its Modifications. 

As has been already noted, the cardiac muscle has features 
peculiar to itself, and occupies histologically an intermediate 
place between the plain and the striped' muscle-cells, and that 
the contraction of the heart is also intermediate in character, 
and is best seen in those forms of the organ which are some- 
what tubular ^nd beat slowly. But the contraction, though 
peristaltic, is more rapid than is usually the case in other 
organs with the smooth form of muscle-fiber. 

The heart behaves under a stimulus in a peculiar manner. 
The effect of a single induction shock depends on the phase of 
contraction in which the heart is at the moment of its applica- 
tion. Thus at the commencement of a systole there is no visi- 
ble effect, while beats of unusual character result at other 
times. But tetanus can not be induced by any form or method 
of stimulation. The latent period of cardiac muscle is long. 

In a heart at rest a single stimulus (as the prick of a needle) 
usually calls forth but one contraction. 

The Nervous System in Relation to the Heart. 

The attempts to determine just why the heart beats at all, 
and especially the share taken by the nervous system, if any 
direct one, are beset with great difficulty ; though, as we shall 
attempt to show later, this subject also has been cramped within 
too narrow limits, and hence regarded in a false light. 

Till comparatively recently the frog's heart alone received 
much attention, if we except those of certain well-known mam- 
mals. In the heart of the frog there are ganglion-cells in vari- 
ous parts, especially numerous in the sinus venosus (or expan- 
sion of the great veins where they meet the auricles) ; also in 
the auricles, more especially in the septum (ganglia of Remak), 
while they are absent from the greater part of the ventricle, 
though found in the auriculo-ventricular groove (ganglia of 

Recently it has been found that ganglion-cells occur in the 
ventricles of warm-blood animals. In the hearts of the dog, 
calf, sheep, and pig, which are those lately subjected to investi- 
gation, it is found that the nerve-cells do not occur near the 
apex of the ventricles, but mainly in the middle and basal por- 
tions, being most abundant in the anterior and posterior inter- 
ventricular furrows and in the left ventricle. But there are 


differences for each group of animals; thus, these ganglion-' 
cells are most abundant, so far as the mammals as yet inves- 
tigated are concerned, in the ventricles of the pig, and least so 
in those of the dog. In the cat they are also scanty. Ganglion- 
cells occur in the auricles, and are especially abundant near the 
terminations of the great veins. 

It has long been known that the heart of a frog removed 
from the body will pulsate for hours, especially if fed with 
serum, blood, or similar fluids ; and that it may be divided in 
almost any conceivable way, even when teased up into minute 
particles, and still continue to beat. The apex, however, when 
separated does not beat. Yet even this quiescent apex may be 
set pulsating if tied upon the end of a tube, through which it 
may be fed under pressure. 

We may here point out that the whole heart or a part of it 
may be made to describe its action by the graphic method in 
various ways, the principles underlying which are either that 
the heart pulls upon a recording lever (lifts it) acts against the 
fluid of a manometer ; or, inclosed in a vessel containing oil or 
similar fluid, moves a piston in a cylinder. 

It has also long been known that a ligature drawn around 
the sinus venosus (in the frog) at its junction with the auricles 
stopped the heart for a certain period, and this experiment (of 
Stannius) was thought to demonstrate that the heart was ar- 
rested because the nervous impulses proceeding to the ganglion- 
cells along the cardiac nerves or ganglia of this region were 
cut off by the ligature ; in other words, the heart ceased to beat 
because the outside machinery on which the action of the inner 
depended was suddenly disconnected. Other explanations have 
been offered of this fact. 

Within the last few years great light has been thrown upon 
the whole subject of cardiac physiology in consequence of in- 
vestigators haying studied the hearts of various cold-blooded 
animals and of several invertebrates. The hearts of the Che- 
lonians (tortoises, turtles) have received special attention, and 
their investigation has been fruitful of results, to the general 
outcome of which, as well as those accruing from recent com- 
parative studies as a whole, we can alone refer. 

Very briefly, the following are some of the main facts : 

1. In all cold-blooded animals the order in which the sub- 
divisions of the heart cease to pulsate when kept under the 
same conditions is invariable, viz., ventricle, auricles, sinus. 

2. The sinus and auricles, when separated by section, liga- 


ture, or otherwise, either together or singly, continue to beat, 
whether amply provided with or surrounded by blood. 

3. The ventricle thus separated displays less tendency to 
beat independent of some stimulus (as feeding under pressure), 
though a very weak one usually suffices — i. e., its tendency to 
spontaneous rhythm is less marked than is the case with the 
other parts of the heart. These remarks apply to the hearts 
of Chelonians — fishes, snakes, and some other cold-blooded 

4. In certain fishes (skate, ray, shark) the beat may be re- 
versed by stimulation, as a prick of the ventricle. This is 
accomplished with more difficulty in other cold-blooded animals, 
and still more so in the mammal. 

5. In certain invertebrates, notably the Poulpe (Octopus), a 
careful search has revealed no nerve-cells, yet their hearts con- 
tinue to beat when their nerves are severed, on section of 
parts of the organ, etc. 

6. A strip of the muscle from the ventricle of the tortoise, 
when placed in a moist chamber and a current of electricity 
passed through it for some hours, will commence to pulsate and 
continue to do so after the current has been withdrawn ; and 
this holds when the strip is wholly free from nerve-cells. 

From the above facts certain inferences have been drawn : 
1. It has been concluded that the sinus is the originator and 
director of the movements of the rest of the heart. 2. That this 
is owing to the ganglia in its walls. While all recognize the 
importance of the sinus, some physiologists hold to the gangli- 
onic influence as essential to the heart-beart, still ; while others, 
influenced by the facts mentioned above, are disposed to regard 
them as of very doubtful importance — at all events, as origina- 
tors of the movements of the heart. 

The tendency now seems to be to attach undue importance 
to the spontaneous contractility of the heart-muscle ; for it by 
no means follows logically that, because a muscle treated by 
electricity, when cut off from the usual nerve influence that we 
believe is being constantly exerted on the heart like other or- 
gans, will contract and continue to do so in the absence of the 
stimulus, it does so normally ; or, because some hearts beat in 
the absence of nerve-cells, that therefore nerve-cells are of no 
account in any case. Such views, when pressed to the extreme, 
lead to as narrow conceptions as those they are intended to re- 

Taking into account the facts mentioned and others we have 


not space to enumerate, we submit the following as a safe view 
to entertain of the beat of the heart in the light of our present 
knowledge : 

Eiecent investigations show clearly that there are great dif- 
ferences in the hearts of animals of diverse groups, so that it 
is not possible to speak of " the heart " as though our remarks 
applied equally to this organ in all groups of animals. 

It must be admitted that our understanding of the hearts of 
the cold-blood animals is greater than of the mammalian heart ; 
while, so far as exact or experimental knowledge is concerned, 
the human heart is the least understood of all, though there is 
evidence of a pathological and clinical kind and subjective 
experience on which to base conclusions possessing a certain 
value ; but it is clear to those who have devoted attention to 
comparative physiology that the more this subject is extended 
the better prepared we shall be for taking a broad and sound 
view of the physiology of the human heart and man's other 

Whatever may be said of the invertebrates, among which 
greater simplicity of mechanism doubtless prevails, there can 
be no doubt that the execution of a cardiac cycle of the heart 
in all vertebrates, and especially in the higher, is a very com- 
plex process from the numbei; of the factors involved, their in- 
teraction, and their normal variation with circumstances ; and 
we must therefore be suspicious of any theory of excessive sim- 
plicity in this as well as other parts of physiology. 

We submit, then, the following as a safe provisional view of 
the causation of the heart-beat : 

1. The factors entering into the causation of the heart-beat 
of all vertebrates as yet examined are : (a) A tendency to spon- 
taneous contraction of the muscle-cells composing the organ ; 
(&) intra-cardiac blood-pressure; (c) condition of nutrition as 
determined directly by the nervous supply of the organ and in- 
directly by the blood. 

2. The tendency to spontaneous contraction of muscle-cells 
is most marked in the oldest parts of the heart (e. g., sinus), 
ancestrally (phylogenetically) considered. 

3. Intra-cardiac pressure exercises an influence in determin- 
ing the origin of pulsation in probably all hearts, though like 
other factors its influence varies with the animal group. In 
the mollusk (and allied forms) and in the fish it seems to be the 
controlling factor. 

4. We must recognize the power one cell has to excite when 


in action neighboring heart-cells to contraction. The ability 
that one protoplasmic cell-mass has to initiate in others, under 
certain circumstances, like conditions with its own, is worthy 
of more serious consideration in health and disease than it has 
yet received. 

5. The influence of the cardiac nerves becomes more pro- 
nounced as we ascend the animal scale. Their share in the 
heart's beat will be considered later. 

6. Apparently in all hearts there is a functional connection 
leading to a regular sequence of beat in the different parts, in 
which the sinus or its representatives (the terminations of great 
veins in the heart) always takes the initiative. One part hav- 
ing contracted, the others must necessarily follow ; hence the 
rapid onset of the ventricular after the auricular contraction in 
the mammal, and the long wave of contraction that seems to 
pass evenly over the whole organ in cold-blooded animals. 

The basis of all these factors is to be sought iinally in the 
natural contractility of protoplasm. A heart in its most devel- 
oped form still retains, so to speak, the inherited but modified 
Amoeba in its every cell. 

Whether the intrinsic nerve -cells of the heart take any 
share directly in the cardiac beat must be considered as yet 
undetermined. Possibly they do modify motor impulses from 
nerves, while again it may be that they have an influence over 
nutritive processes only. The subject requires further study, 
both anatomical and physiological. 

Influence of the Vagus Nerve upon the Heart. 

The principal facts in this connection may be stated as fol- 
lows, and apply to all the animals thus far examined : 

1. In all cases the action of the heart is modified by stimu- 
lation of the medulla oblongata or the vagus nerve. 

2. The modification may consist in prompt arrest of the 
heart, in slowing, in enfeeblement of the beat, or a combination 
of the two latter effects. 

3. After the application of the stimulation there is a latent 
period before the effect is manifest, and the latter may outlast 
the stimulation by a considerable period. 

4. In most animals the sinus venosus and auricles are af- 
fected before the ventricles, and the vagus may influence these 
parts when it is powerless over the ventricle. 

6. After vagus inhibition, the action of the heart is (almost 



unexceptionally) different, the precise result being variable, but 
generally the beat is both accelerated and increased in force. 



Fir. S38.— Inhibition of frog's heart by stimulation of the vagus nerve. To be read from right 
to left. The contractions of the ventricle are registered by a simple lever resting on it. ■ 
The interrupted current was thrown in at a. Note that one beat occurred before arrest 
(latent period), and that when standstill of the heart did take place it lasted for a consider- 
able period (Foster). 

We may say that the working capacity of the heart is tem- 
porarily increased. 

6. The improvement in the efficiency of the heart is in pro- 
portion to its previous working power, and in cases when the 

Stimulatinn VaguH. 

Fio. 839.— Effects of vagus stimulation, illustrated by a form of sphygmographic curve derived 
from the carotid of a rabbit (Foster). 

action is feeble and irregular (abnormal) it might be said to be 
in proportion to its needs. This is a very important law that 
deserves to receive a general recognition. 

7. Section of both vagi nerves results in histological altera- 
tions in the heart's structure, chiefly fatty degeneration, which 
must, of course, impair its working capacity and expose it 
to rupture or other accidents under the frequently recurring 
strains of life. 


8. In the cold-blooded animals the heart may be kept at a 
standstill by vagus stimulation till it dies, a period of hours 
(one case of six hours reported for the sea-turtle). 

9. Certain drugs (as atropine), applied directly to the heart, 
or injected into the blood, prevent the usual action of the 

10. During vagus arrest the heart substance undergoes a 
change, resulting in an unusual dilatation of the organ. This 
may be witnessed whether the heart contains blood or not. 

11. The heart may be arrested by direct stimulation, espe- 
cially of the sinus, and at the points at which the electrodes are 
applied there is apparently a temporary paralysis. The same 
alteration in the beat may be noticed, as when the main trunk 
of the vagus is stimulated. 

12. The heart may be inhibited through stimulation of vari- 
ous parts of the body, both of the surface and internal organs 
(reflex inhibition). 

13. One vagus being divided, stimulation of its upper end 
may cause arrest of the heart. 

14. Stimulation of a small part of the medulla oblongata 
will produce the same result, provided one or both vagi be 

15. Section of both vagi in some animals (the dog notably) 
increases the rate of the cardiac beat. The result of section of 
one pneumogastric nerve is variable. The heart's rhythm is 
usually to some extent quickened. 

16. During vagus inhibition from any cause in mammals 
and many other animals, the heart responds to a single stimu- 
lus, as the prick of a needle, by at least one beat. An observer 
studying for himself the behavior of the heart in several groups 
of animals with an open mind, for the purpose of observing 
all he can rather than proving or disproving some one point, 
becomes strongly impressed with the variety in unity that runs 
through cardiac physiology, including the influence of nerve- 
cells (centers) through nerves ; for it will not be forgotten that 
normally nerves originate nothing, being conductors only, so 
that when the vagus is stimulated by us we are at the most 
but imitating in a rough way the work of central nerve-cells. 
We can only mention a few points to illustrate this. 

In the frog a succession of light taps, or a single sharp one 
(" Klopf versuch " of Goltz), will usually arrest the heart re- 
flexly; though sometimes it is very difficult to accomplish. 
But in the fish the ease with which the heart may be reflexly 



inhibited by gentle stimulation of almost any portion of tlie 
animal is wonderful. Again, in some animals the vagus arrests 

E. Vagus. 


Brain above Medulla. 

Cardio-inhibitory Center 
in Medulla Oblongata. 

Afferent Nerve. 

Outlying Area with its 

Fig. 240.— Diagram of the inhibitory mechanism of the heart. The arrows indicate in all 
C£ises the path the nervous impulses t&ke. I. Path of afferent impulses from the heart 
itself, n. Path from parts of the brain above (or anterior to) the vaso-motor center. A 
similar one might, of course, be mapped out along the spinal cord. III. Path from some 
peripheral region. The downward arrows indicate the course of efEerent impulses, which 
« probably \isually pass by both vagi. 

the heart for only a brief period, when it breaks away into its 
usual (but increased) action. 

In the fish, menobranchus, and probably other animals, the 
irritability of some subdivision of the heart is lost during the 
vagus inhibition — i. e., it does not respond to a mechanical 

There is usually a certain order in which the heart recom- 
mences after inhibition (viz., sinus, auricles, ventricles) ; but 
there are. variations in this, also, for different animals.' It is 


also a fact that in most of the cold-blooded animals the right 
vagus is more efficient than the left, owing, we think, not to the 
nerves themselves so much as to their manner of distribution 
in the heart — the greater portion of the driving part of the 
organ, so to speak, being supplied by the right nerve ; for, when 
even a small part of the heart is arrested, it may be overcome 
by the action of a larger portion of the same, or a more domi- 
nant region (the sinus mostly). 

Conclusions. — The inferences from the facts stated in the 
above paragraphs are these : 1. There is in the medulla a col- 
lection of cells (center) which can generate impulses that reach 
the heart by the vagi nerves and influence its muscular tissue, 
though whether directly or through the intermediation of 
nerve-cells in its substance is uncertain. It may possibly be in 
both ways. 2. This center (cardio-inhibitory) may be influ- 
enced reflexly by influences ascending by a variety of nerves 
from th« periphery, including paths in the brain itself, as 
shown by the influence of emotions or the behavior of the 
heart. 3. The cardio-inhibitory center is the agent, in part, 
through which the rhythm of the heart is adapted to the needs 
of the body. 4. The arrest, on direct stimulation of the heart, 
is owing to the effect produced on the terminal fibers of the 
vagi, as shown by the dilation, etc., corresponding to what 
takes place when the trunk of the nerve or the center is stimu- 
lated. 5. The quickening of the heart, following section of the 
vagi, seems to show that in some animals the inhibitory center 
exercises a constant regulative influence over the rhythm of 
the heart. 6. The irritability and dilatability of the cardiac 
tissue may be greatly modified during vagus inhibition. Some- 
times this is evident before the rhythm itself is appreciably 
altered. 7. The heart-muscle has a latent period, like other 
kinds of muscle ; and cardiac effects, when initiated, last a 
variable period. 

There are many other obvious conclusions, which the stu- 
dent will draw for himself. 

But a question arises in regard to the significance of the 
cardiac arrest under these circumstances, and the altered action 
that follows. The fact that, when the heart is severed from the 
central nervous system by section of its nerves, profound 
changes in the minute structure of its cells ensue, points un- 
mistakably to some nutritive influence that must have operated 
through the vagi nerves. That stimulation of the vagus re- 
stores regularity of rhythm and strengthens the beat of the 


failing heart, is also very suggestive. That many disorders of 
the heart are coincident with periods of mental anguish or 
worry, and that in certain cases of severe mental application 
the heart's rhythm has become very slow, also point to influ- 
ences of a central origin as greatly affecting the life-processes 
of this organ. 

It has been shown that the vagus nerve in some cold-blooded 
animals, as is probable also in the higher vertebrates, consists 
of two sets of fibers — those which are inhibitory proper and 
those which are not, but belong to the sympathetic system. 

Separate stimulation of the former favors nutritive pro- 
cesses, is preservative; of the latter, destructive. This has 
been expressed by saying that the former favors constructive 
(anabolic) metabolism ; the latter destructive (katabolic) me- 
tabolism. It is assumed that all the metabolism of the body 
may be represented as made up of katabolic following anabolic 

Whether such a view of metabolism expresses any more 
than a sort of general tendency of the chemistry of the body 
is doubtful. It is a very simple representation of what in all 
probability is extremely complex; and if it be implied that 
throughout the body certain steps are always taken upward in 
construction to be always afterwards followed by certain down- 
ward destructive changes, we must reject it as too rigid and 
artificial a representation of natural processes. 

We think, however, that, upon all the evidence, pathological 
and clinical as well as physiological, the student may believe 
that the vagus nerve, like the other nerves of the body, accord- 
ing to our own theory, exercises a constant beneficial, guiding 
— let us say determining — influence over the metabolism of the 
organ it supplies ; and we here suggest that, if this view were 
applied to the origin and course of cardiac disease, it would 
result in a gain to the science and art of medicine. 

The Accblbkator (Augmbntor) Nbrvbs of the Heart. 

It has been known for many years that in the dog, cat, rab- 
bit, and some other mammals, there were nerves proceeding 
from Certain of the ganglia of the sympathetic chain high up, 
stimulation of which led to an acceleration of the heart-beat. 
Very recently these nerves have been traced in a number of 
cold-blooded animals, and the whole subject placed on a broader 
and sounder basis. 



There are variations in the distribution of these nerves for 
different groups of animals, but it will suffice if we indicate 
their course in a general way, without special reference to the 
variations for each animal group : 1. These nerves emerge from 
the spinal cord (upper dorsal region), and proceed upward 
before being distributed to the heart. 2. They may leave for 
their cardiac destination either at (a) the first thoracic (or basal 
cardiac ganglion, as it might be named in this case), (b) the in- 
ferior cervical ganglion, (c) the annulus of Vieussens, or (d) the 
middle cervical ganglion. 

Spinal Cord. 

Accelerator Center in Medulla. 

Superior Cervical Ganglion. 

2 n Middle Cervical Ganglion. 

Interior Cervical Ganglion. 

Region of First Rib. 

Accelerator Nerves. 


Fig. 241. — Diagram to illustrate the origin, course, etc., of accelerator impulses. It will be 
understood that this is intended to indicate the general plan, and not i)reciselr what takes 
place in any one animal. Thus, while the accelerator nerves may arise in this way, it is 
not meant to be implied that the heart is actually supplied by three nerves of such origin 
-in any case. The arrows, as before, indicate the path of the impulses. 

Their course has been traced by physiological methods ; thus 
it has been found that, in all animals examined, stimulation 
of the spinal cord or the various parts mentioned above, or 
nerve branches from them, gave rise either to acceleration of 


the cardiac beat or augmentation of its force, or to both, as is 
commonly the case. In every instance the work of the heart 
is increased, so that they may be called more appropriately 
augmentor nerves; and their efEect may be more evident on 
one part of the heart, as regards increase of the force of the 
beat, than on another. 

They require for their fullest effect a rather strong and con- 
tinuous stimulation (interrupted current), and the augmenta- 
tion outlasts the stimulus a considerable period. The same law 
applies to them as to the vagus nerve, viz., that the result is 
inversely proportional to the rhythm of the heart at the period 
of stimulation ; a slow-beating heart will be more augmented 
proportionally than a rapidly-pulsating organ. 

It is noticeable that after one or more experiments the heart 
often falls into an irregular or weakened action quite the re- 
verse of what ensues when the vagus is stimulated. But it has 
also been observed that certain of the vagus fibers on stimula- 
tion give rise to a like result. 

Further, it is found that the electrical condition of the heart 
is different, according as the inhibitory or other fibers of the 
heart are stimulated. The latter fact seemed to point strongly 
to a fundamental difference in their effect on cardiac metabo- 
lism; hence it is proposed to speak of the vagus as a vago- 
sympathetic nerve, containing inhibitory fibers proper and 
sympathetic or motor fibers to be classed with the nerves that 
were formerly known as " accelerators," and to be compared 
in their action to the ordinary motor nerves of voluntary 

Indeed, these conceptions will probably give rise to a broader 
view of the whole nervous system, especially as regards the 
relations of the nerves themselves. 

Certainly the augmentor nerves to which we are now refer- 
ring exhaust the heart, lead it to expend its nutritive capital, 
and leave it worse than before. One can understand the ad- 
vantage in the heart having a double supply of nerve-fibers 
with opposite action ; and it is worthy of special note in this 
connection that, when the vagus (vago-sympathetic) is stimu- 
lated at the same time as the augmentors, the inhibitory effect, 
preservative of nutritive resources, prevails. 

It will be seen that the heart may be made to do increased 
work in three ways : Firstly, the relaxation of a normal inhibi- 
tory control through the vagus nerve by the cardio-inhibitory 
center; secondly, through the sympathetic (motor) fibers in 


the vagus itself ; and, finally, through, fibers with similar action 
in the sympathetic system, usually so called. 

The share taken by these factors is certainly variable in dif- 
ferent species of animals, and it is likely that this is true of the 
same animals on different occasions. It is also conceivable, 
and indeed probable, that they act together at times, the inhibi- 
tory action being diminished and the augmentor influence in- 

Human Physiology. — Of the three cardiac nerves — superior, 
middle, and inferior — ^the strongest, which is the middle one, 
passes from the inferior cervical ganglion to the middle, from 
which it proceeds to the heart, and the inferior, may be re- 
garded as the chief augmentor cardiac nerves. 

That man's pneumogastric contains inhibitory fibers is evi- 
dent from the experiment of Czermak, who, by pressing a bony 
tumor in his aeck against his vagus nerve, could arrest his 
heart. Another individual could arrest his heart-beat at will, 
and if not through the vagus, how ? 

We are probably all aware of alterations in the rhythm of 
the heart from emotions. During a period of intense, brief, 
sympathetic anxiety, as in watching two competitors during a 
severe struggle for supremacy, a change in the rhythm of the 
heart, amounting, it may be, to momentary arrest, may be 

Enough has been said, we trust, to show that the nerves of 
the heart can no longer be regarded merely as the reins for 
bridling the cardiac steed; but that all the phenomena of accel- 
eration, slowing, or other changes of rhythm, are only the out- 
ward evidences of profound vital changes accompanied by cor- 
responding chemical and electrical effects. If these views be 
correct, nervous influence must play no small part in the causa- 
tion and modification of disordered conditions ; and we would 
extend such a view to all the organs of the body, and especially 
in the case of man. The heart's rhythm can, however, be 
modified in other ways than we have as yet described. 

Though an isolated heart, fed by serum or some artificial 
nutritive fluid, may beat well for a time, it is liable to peri- 
odic interruptions, which are probably owing to its imperfect 

Many drugs greatly modify the heart-beat ; but, in attempt- 
ing to explain how the result is accomplished, the difficulty is 
in unraveling the part each anatomical element plays in the 
total result. Does the drug act on the muscular tissue, the 



nerve terminals, or the ganglia; or does it affect the heart 
through the central nervous system ? 

Resort to comparative physiology is important in such cases, 
if only to foster caution and avoid narrow views. 

The Heart in Relation to Blood- Pressure. 

It is plain that all the other conditions throughout the cir- 
culatory system remaining the same, an increase in either the 
force or the frequency of the heart-beat must raise the blood-press- 
ure. But, if the pressure were generally raised when the heart 
beats rapidly, it would fare ill with the aged, the elasticity of 
their arteries being usually greatly impaired. As a matter of 
fact any marked rise of pressure that would thus occur is pre- 
vented as a rule, and in different ways, as will be seen ; but, so 
far as the heart is concerned, its beat is usually the weaker the 
more rapid it is, so that the cardiac rhythm and the blood- 
pressure are in inverse proportion to each other. 

By what method is the heart's action tempered to the condi- 
tions prevailing at the time in the other parts of the vascular 
system ? 

The matter i-s complex. It is possible to conceive that there 
is a local nervous apparatus which regulates the beat of the 
heart according to the intra-cardiac pressure, which latter again 
will depend on conditions outside of the heart itself — the arte- 
rial pressure, in fact. It is possible to understand that, apart 
- from any nervous elements at all, the cardiac cells regulate 
their own action in obedience to the impressions made upon 

But, inasmuch as the heart is not regulated perfectly in the 
mammal according to the blood -pressure, when the vagi nerves 
are cut, and considering the dominance of the central nervous 
system, it does not seem likely that it should resign the con- 
trol of so important a matter. Experiment bears this out. 
There is some evidence for believing that not only may the 
vagus itself act as an afferent sensory nerve, but that the de- 
pressor nerve, to be shortly referred to more particularly, is 
also such a sensory nerve. 

However, such a view does not exclude previously men- 
tioned factors, and there can be little doubt that in forms below 
mammals the muscular tissue is to some degree self -regulative ; 
and it is not likely that this quality is wholly lost even in the 
highest mammals. 




The effect of vagus stimulation on the blood-pressure is 
always very marked, as would be supposed. To examine an 
extreme case, suppose the heart arrested for a few seconds, the 
elastic recoil of the arteries continues to maintain for a time 
the blood-pressure, though there is, of course, an immediate 
and pronounced fall. And it may bfe remarked, by-the-way, 
that in cases of fainting, when the heart ceases to beat, or beats 
in the feeblest man- 
ner, the importance 
of this arterial elas- 
ticity as a force, 
maintaining the 
circulation for sev- 
eral seconds at 
least, is of great 

As seen in the 
tracing, the beats, 
when the heart 
commences its ac- 
tion again, tell on 
the comparatively 
slack walls of the 
arteries, distending them greatly, and this may be made evident 
by the sphygmograph as well as the manometer ; indeed, may 
be evident to the finger, the pulse resembling in some features 
that following excessive loss of blood. 

If the heart has been ■ merely slowed, or its pulsation weak- 
ened, the effects will of course be less marked. 

The Quantity of Blood. — The blood-pressure may also be 
augmented, the cardiac frequency remaining the same, by 
the quantity of blood ejected from the ventricles, which again 
depends on the quantity entering them, a factor determined 
by the condition of the vessels, and to this we shall presently 

In consequence of changes in different parts of the system 
by way of compensation, results follow in an animal which 
might not have been anticipated. 

Thus, bleeding, unless to a dangerous extreme, does not 
lower the blood-pressure except temporarily. It is estimated 
that the body can adapt itself to a loss of as much as 3 per 
cent of the body-weight. 

The adaptation is probably not through absorption chiefly. 

, ng t. 

diac inhibition on blood-pressure. The fall in this case 
was very rapid, owing to sudden cessation of the hearts 
beat. The relative emptiness of the vessels accounts for 
the peculiar character of the curve of rising blood-pressure 


but through, constriction of the vessels by the vaso-motor 

Again, an injection of fluid into the blood does not cause an 
appreciable rise of blood-pressure, so long as the nervous sys- 
tem is intact ; but, if by section of the spinal cord the vaso- 
motor influences are cut off, then a rise may take place to the 
extent of 2 to 3 per cent of the body -weight, the extra quan- 
tity of fluid seeming to be accommodated in the capillaries and 
smaller veins. These facts are highly significant in illustrat- 
ing the adaptive power of the circulatory system (protective in 
its nature), and are of practical importance in the treatment of 

We think the benefit that sometimes follows bleeding has 
not as yet received an adequate explanation, but we shall not 
attempt to tackle the problem now. Changes in the circulation 
depend on variations in the size of the blood-vessels. 

It is important in considering this subject to have clear no- 
tion's of the structure of the blood-vessels. It will be borne in 
mind that, while muscular elements are perhaps not wholly 
lacking in any of the arteries, they are most abundant in the 
smallest, the arterioles, which by their variations in size are 
best fitted to determine the quantity of blood reaching any 
organ. It is well known that nerves derived chiefly from the 
sympathetic system pass to blood-vessels, though their exact 
mode of termination is obscure. 

We may now examine into the nature of certain facts, which 
may be stated briefly thus : 

1. In certain vascular areas of some vertebrates, as in the 
vessels of the ear of the rabbit and this animal's saphena 
artery, rhythmical variations in the size of the small arteries 
may be observed ; also in the veins of the bat's wing and of the 
fins of certain fishes (e. g., caudal vein of the eel), as well as in 
certain arteries of some groups of the cold-blooded animals. 

2. Under the microscope the arterioles of various parts of 
the frog, including those of the muscles, may be seen to vary 
apparently spontaneously, and may through stimulation be 
made to depart widely from their usual size. 

3. Section of a large number of nerves is followed by red- 
dening of the parts to which they are distributed. This is well 
seen when the cervical sympathetic of the rabbit is divided ; the 
ear becomes redder, owing to obvious dilatation of its blood- 
vessels ; and warmer, owing to the increased quantity of blood 
in it, etc. It has also been noticed in cases of paralysis, and 


especially in gunsliot and other "wounds involving nerves, that 
vaso-motor effects have followed. 

4. Section of certain nerves, as the nervi erigentes of the 
penis, is not followed by dilatation ; but these nerves and the 
chorda tympani supplying the salivary gland are examples of 
so-called vaso-dilators, inasmuch as their stimulation gives rise 
to enlargement of the caliber of the arterioles in their area of 

5. On the other hand, such a nerve as the cervical sympa- 
thetic, as may be readily shown in the rabbit, when its periph- 
eral end is stimulated, gives rise to constriction, and hence is 
termed a vaso-constrictor. 

6. When, however, the divided sciatic nerve is stimulated 
peripherally, the result may be either constriction or dilata- 

7. When the spinal cord of an animal is divided across, 
there is vascular dilatation of all the parts below the section 
(loss of arterial tone) ; but in time the vessels return to their 
usual size (restoration of arterial tone). 

8. On destruction of a certain minute portion of the medulla 
oblongata, there is a general loss of arterial tone. This area 
(center) extends in the rabbit from a short distance below the 
corpora quadrigemina (1 to 3 mm.) to within 4 to 5 of the 
calamus scriptorius, as ascertained by the effects on the vessels 
of cutting away the medulla in thin transverse sections. At 
the spot indicated there is a collection of large multipolar 
nerve-cells (antero-lateral nucleus of Clarke). 

Conclusions. — 1. There 'are vaso-motor nerves of two kinds — 
vaso-constrictors and vaso-dilators — which may exist in nerve- 
trunks either alone or mingled. 

Examples of the former are found in the cervical sympa- 
thetic, splanchnic, etc., of the latter in the chorda tympani, 
nerves of the muscles and nervi erigentes (from the first, second, 
and third sacral nerves), while the sciatic seems to contain 
both. 2. Impulses are constantly passing from the medullary 
vaso-motor center along the nerves to the blood-vessels, hence 
their dilatation after section of the nerves. 

The nerves are traceable to the spinal cord, and in some 
part of their course run, as a rule, in the sympathetic system. 
3. Impulses pass at intervals to the areas of distribution of 
vaso-dilators along these nerves, the effect of which is to dilate 
the vessels through their influence, as in other cases, on the 
muscular coat. 



It ia stated that in course of time the vessels of the rabbit's 
ear regain their tone, notwithstanding that the influence of the 

Spinal Cord 

Vaso-motor Center in 

-Depressor Nerve. 

Efferent Vaso-raotor 

Outlying Vascular 

Afferent Nerve from 

Fig. 243.— Diajgram of nervous vaso-motor mechanism. I. Course of afferent impulses from 
the heart itself along the depressor nerve, n. Course from some other part of the brain, 
m. Course from some peripheral region along a nerve joining the spinal cord. The effer- 
ent impulses are represented as passing to a vascular area, reduced for the sake of sim- 
plicity to a single arteriole. 

central nervous system has been cut off by section of the vaso- 
motor nerves. 

To explain this result, a local nervous mechanism has been 
assumed to exist, though not demonstrated either anatomically 
or physiologically. Interesting experiments have lately shown 
that both in mammals and cold-blooded animals the effect on 
the blood-vessels varies with the intensity and character of the 
stimulus, and not only with the group of animals tested, but 
even with the same individuals at different periods during the 
experiment ; and we take the opportunity to renew our expres- 
sion of opinion with this fresh evidence that the laws of physi- 
ology can not be laid down in the rigid way that has prevailed 


to SO large an extent np to the present time ; but that our widen- 
ing experience shows (what ought to have been expected) that 
the greatest allowance mast be made for group if not individ- 
ual variations everywhere. There is also evidence to show that 
the mode of stimulation in experimental cases causes the result 
to vary. From such facts as are stated in paragraph seven, it 
is inferred that there are vaso-motor centers in the spinal cord 
which are usually subordinated to the main center in the me- 
dulla, but which in the absence of the control of the chief cen- 
ter in the medulla assume an independent regulating influence. 

A local vaso-motor mechanism does not seem to us neces- 
sary to explain the changes which the blood-vessels undergo, 
and should not be adopted as an article of physiological faith 
till demonstrated to exist. If we assume that the independent 
contractility of muscle-cells is retained in the blood-vessels, 
and that, when freed from the influence of the central nervous 
system, which becomes more and more dominant as we ascend 
the animal scale, there is a reversion to an ancestral condition, 
a new light is thrown upon the facts. It is a case of old habits 
gaining sway when the check-rein of nervous influence is re- 
moved ; and, as we shall show from time to time, this law applies 
to every organ of the body. Moreover, not to go beyond the 
vascular system, this independent rhythmic activity is seen in 
the isolated sections of the pulsatile veins of the bat's wing, 
devoid, so far as we know, of nervous cells. Such facts lend 
some color to the view that, after distention of the. vessels by 
the cardiac systole, the return to their previous size is aided by 
rhythmical contractions of the muscle-cells. 

Let us now consider certain other well-known experimental 
facts : 

1. There is a nerve with variable origin, course, etc., in dif- 
ferent mammals, but in the rabbit given off from either the 
vagus, the superior laryngeal, or by a branch from each, 
which, running near the sympathetic nerve and the carotid 
artery, reaches the heart, to which it is distributed. This is 
known as the depressor nerve. 

2. The vagi nerves having been divided, stimulation of the 
central end of the cut depressor nerve is followed by a fall in 
blood-pressure, which may not be accompanied by any altera- 
tion in the cardiac rhythm. 

3. This effect may in great part be prevented if the splanch- 
nic nerves be divided previous to stimulation of the depressor. 

4. If the splanchnic area (region of the main abdominal 



viscera) be inspected during the fall in blood-pressure, it may- 
be noticed that there is vascular fullness under these circum- 

These results are interpreted as being due to afferent im- 
pulses ascending the depressor, acting on the vaso-motor center^ 


Fig. 244. — Curre of blood-pressure resulting from stimulation of the central end of the de- 
pressor nerve. To be read from right to left. T indicates the rate at which the recording 
surface moved, the intervals denoting seconds. At C the current was thrown into the 
nerve, and shut off at O. The result appears after a period of latency, and outlasts the 
stimvilus (Foster). 

and interfering with (inhibiting) the outflow of efferent, con- 
strictive, or tonic impulses, which start from the vaso-motor 
center, descend the cord, and find their way to the organs of 
the region in question, in consequence of which the mus- 
cular coats of the arterioles relax, more blood flows to this 
area which is very large, and the general blood-pressure is 

Again, if the central end of one of the main nerves — e. g., 
sciatic — be stimulated, a marked change in the blood-pressure 
results, but whether in the direction of rise or fall seems to 
depend upon the condition of the central nervous system, for, 
with the animal under the influence of chloral, there is a fall ; 
if under urari, a rise. 

It is not to be supposed that the change in any of these 
cases is confined to any one vascular area invariably, but that 
it is this or that, according to the nerve stimulated, the condi- 
tion of the centers, and a number of other circumstances. 
Moreover, it is important to bear in mind that with a fall of 
blood-pressure in one region there may be a corresponding rise 
in another. "With these considerations in mind, it will be ap- 
parent that the changes in the vascular system during the 


course of a single hour are of tie most complex and variable 

Though special attention has been drawn to such rhyth- 
mical variations as may be witnessed in the rabbit's ear, bat's 
wing, etc., there can be little doubt that changes as marked, 
though possibly less distinctly rhythmical, are constantly tak- 
ing place in the vertebrate body, and especially in that of man, 
wijth his complex emotional nature and the many vicissitudes 
of modern civilized life. The frequent changes in color in the 
faces of certain people are in this connection suggestive, though 
we hope we have made it clear that these vascular modifica- 
tions are dependent chiefly on centripetal influences from every 
quarter, though actually brought about by centrifugal im- 
pulses. Whether there is a rhythm obscured by minor rhythms, 
owing to an independent or automatic action of the vaso-motor 
center, though not improbable, must be regarded as undeter- 
mined as yet. 

The question of the distribution of vaso-motor nerves to 
veins is also one to which a definite answer can not be given. 

The Capillaries. 

The cells of which the capillaries are composed have a con- 
tractility of their own, and hence the caliber of the capillaries 
is not determined merely by the arterial pressure or any similar 
mechanical effect. 

Certain abnormal conditions, induced in these vessels by 
the application of irritants, cause changes in the blood-flow, 
which can not be explained apart from the vitality of the ves- 
sels themselves. 

Watched through the microscope under such circumstances, 
the blood-corpuscles no longer pursue their usual course in the 
mid-stream, but seem to be generally distributed and to hug the 
walls, one result of which is a slowing of the stream, wholly 
independent of events taking place in other vessels. It is thus 
seen that in this condition (stasis) the capillaries have an in- 
dependent influence essentially vital. We say independent, for 
it is still an open question whether nerves are distributed to 
capillaries or not. That inflammation, in which also the walls 
undergo such serious changes that white and even red blood- 
cells may pass through them (diapedesis), is not uninfluenced 
by the nervous system, possibly induced through it in certain 
cases, if not all, seems more than probable. 


But wieii we consider the lymphatic system new light will, 
it is hoped, be thrown upon the subject of the nature and the 
influences which modify the capillaries. One thing will be 
clear from what has been said, that even normally the capil- 
laries must exert an influence of the nature of a resistance, 
owing to their peculiar vital properties ; and, as we have 
already intimated, such considerations should not be excluded 
from any conclusions we may draw in regard to tubes that are 
made up of living cells, whether arteries, veins, or capillaries, 
though manifestly the applicability to capillaries with their 
less modified or more primitive structure is stronger. 

It has now become clear that the circulation may be modi- 
fied either centrally or peripherally ; that a change is never 
purely local, but is correlated with other changes ; that the 
whole is, in the higher animals, directly under the dominion 
of the central nervous system ; and that it is through this 
part chiefly that harmony in the vascular as in other sys- 
tems and with other systems is established. To have ade- 
quately grasped this conception is worth more than a knowl- 
edge of all the details. 

Special Considerations. 

Pathological. — Changes may take place either in the sub- 
stance of the cardiac muscles, in the valves, or in the blood-ves- 
sels, of a nature unfavorable to the welfare of the body. Some 
of these have been incidentally referred to already. 

Hypertrophy, or an increase in the tissue of the heart, is 
generally dependent on increased resistance, either within or 
without the heart, in the region of the arterioles or capillaries. 
Imperfections of the aortic valves may permit of regurgitation 
of blood, entailing an extra eif ort if it is to be expelled in addi- 
tion to the usual quantity, which again leads to hypertrophy ; 
but this is often succeeded by dilatation of the chambers of the 
heart one after the other, and a host of evils growing out of 
this, largely dependent on imperfect venous circulation, and 
increased venous pressure. And it may be here noticed that 
arterial and venous pressures are, as a general rule, in inverse 
proportion to each other. 

If the quantity of blood in the ventricle, in consequence 
of regurgitation, should prove to be greater than it can lift 
(eject), the heart ceases to beat in diastole ; hence some of the 
sudden deaths from disease of the aortic valves. 


As a result of fatty, or other forms of degeneration, tlie 
heart may suddenly rupture under strains. 

Actual experiment on the arteries of animals recently dead, 
including men, shows that the elasticity of the arteries of even 
adult mammals is as perfect as that of the vessels of the child, 
so that man ranks lower than other animals in this respect. 

After middle life the loss of arterial elasticity is consider- 
able and progressive. The arteries may undergo a degenera- 
tion from fatty changes or deposit of lime ; such vessels are, of 
course, liable to rupture ; hence one of the frequent modes of 
death among old persons is from paralysis traceable to rupture 
of vessels in the brain. 

These and other changes also cause the heart more work, 
and may lead to hypertrophy. Even in young persons the 
strain of a prolonged athletic career may entail hypertrophy 
or some other form of heart-disease. 

We mention such facts as these to show the more clearly 
how important is balance and the power of ready adaptation 
in all parts of the circulation to the maintenance of a healthy 
condition of body. 

The heart is itself nourished through the coronary arteries ; 
so that morbi^ alterations in these vessels cause, if not sudden 
and painful death, at least nutritive changes in the heart-sub- 
stance, which may lead to a dramatic end or to a slow impair- 
ment of cardiac power, etc. 

Personal Observation. — The circulation is one of those depart- 
ments of physiology in which the student may verify much upon 
his own person. ■ The cardiac impulse, the heart's sounds (with a 
double stethoscope), the pulse — its nature and changes with cir- 
cumstances, the venous circulation, and many other subjects, 
are all easy of observation, and after a little practice without 
liability of causing those aberrations due to the attention being 
drawn to one's self. 

The observations need not, of course, be confined to the stu- 
dent's own person ; it is, however, very important that the nor- 
mal should be known before the observer is introduced to cases 
of disease. Frequent comparison of the natural and the dis- 
eased condition renders physiology, pathology, and clinical 
medicine much good service. We again urge upon the student 
to try to form increasingly vivid and correct mental pictures 
of the circulation under its many changes. 

Comparative. — An interesting arrangement of blood-vessels, 
known as a rete mirdbile, occurs in every main group of verte- 



brates. An artery breaks up into a great number of vessels of 
nearly the same size, wMcli terminate, abruptly and without 
capillaries, in another arterial trunk. 

Fig. a^.—Rete mirabile of sheep, seen in profile (after Chauveau). The larger rete is in con 
nection with the encephalic arteries ; the smaller, the ophthalmic. The large artery is the 

They are found in a variety of situations, as on the carotid 
and vertebrate arteries of animals that naturally feed from the 
ground for long periods together, as the ruminants; in the 

Fict. 246.— Section of a lymphatic rete mirabile, from the popliteal space (after Cfhauveau). 
a, a, afferent vessels ; o, 6, efferent vessels. The "whole very strongly suggests a crude 
form of lymphatic gland. 



sloth, that hangs from trees ; in the legs of swans, geese, etc. ; in 
the horse's foot, in -which the arteries break up into many small 
divisions. It has been 
suggested that these ar- 
rangements permit of a 
supply of arterial blood 
being maintained without 
congestion of the parts. 
Very marked tortuosity 
of vessels, as in the seal, 
the carotid of which is 
said to be forty times as 
long as the space it trav- 
erses, in all probability 
serves the same purpose. 

Evolntion. — The com- 
parative sketch we have 
given of the vascular sys- 
tem will doubtless sug- 
gest a gradual evolution. 
We observe throughout a 
dependence and resem- 
blance which we think 
can not be otherwise ex- 
plained. The similarity 
of the foetal circulation in the mammal to the permanent circu- 
lation of lower groups has much meaning. Even in the high- 
est form of heart the original pulsatile tube is not lost. The 
great veins still contract in the mammal ; the sinus venosus is 
probably the result of blending and expansion. The later 
differentiations of the parts of the heart are clearly related to 
the adaptation to altered surroundings. Such is seen in the 
foetal heart and circulation, and has probably been the deter- 
mining cause of the forms which the circulatory organs have 

It is a fact that the part of the heart that survives the long- 
est under adverse conditions is that which bears the stamp of 
greatest ancestral antiquity. It (the sinus venosus) may not 
be less under nervous control, but it certainly is least depend- 
ent on the nervous system, and has the greatest automaticity. 

It is surely fortunate for man that this part of the reptilian 
heart is represented in his own. In cases of fainting, partial 
drowning, or other instances of impending death, this part, with 

Fig. 247.- 

Veins of the toot of the horse (after Chau- 


tke auricles it may be, continues to beat when the ventricles 
have ceased ; and we have learned that so long as these parts 
are functionally active there is a greater probability that the 
quiescent regions may recommence. Activity begets activity, 
in cardiac muscle-cells at least. How are these facts to be 
explained apart from evolution ? 

The law of rhythm in organic nature finds some of its most 
evident exemplifications in the circulation. Most of the 
rhythms are compound, one being blended with or superim- 
posed on another. Even the apparent irregularities of the nor- 
mal heart are rhythmical, such as the very marked slowing 
and other changes accompanying expiration, especially in some 

We trust we have made it evident that the greatest allow- 
ance must be made for the animal group, and some even for 
the individual, in estimating any one of the factors of the cir- 
culation. We know a good deal at present of cardiac physiol- 
ogy, but Ave do not know a physiology of " the heart " in the 
sense in which we understand that term to have been used till 
recently — i. e., we are not in a position to state the laws that 
apply to all forms of heart. 

Summary of the Physiology of the Circulation. — In the mammal 
the circulatory apparatus forms a closed system consisting of a 
central pump or heart, arteries, capillaries, and veins. All the 
parts of the vasciilar system are elastic, but this property is 
most developed in the arteries. 

Since the tissue-lymph is prepared from the blood in the 
capillaries, it may be said that the whole circulatory system 
exists for these vessels. 

As a result of the action of an intermittent pump on elastic 
vessels against peripheral resistance, in consequence of which 
the arteries are always kept more than full (distended), the 
flow through the capillaries and veins is constant — a very great 
advantage, enabling the capillaries to accomplish their work of 
feeding the ever-hungry tissues. While physical forces play a 
very prominent part in the circulation of the blood, vital ones 
must not be ignored. They lie at the foundation of the whole, 
here as elsewhere, and must be taken into the account in every 

As a consequence of the anatomical, physical, and vital char- 
acters of the circulatory system, it follows that the velocity of 
the blood is greatest in the arteries, least in the capillaries, and 
intermediate in the veins. 


The veins with their valves, their superficial position and 
thinner walls, make np a set of conditions favoring the onflow 
of the blood, especially under muscular exercise. 

In the mammal the circulatory system, by reason of its con- 
nections with the digestive, inspiratory, and lymphatic systems, 
and in a lesser degree with all parts of the body, especially the 
glandular organs, maintains at once the usefulness and the fit- 
ness of the blood. 

The arterioles, by virtue of their highly developed muscular 
coat, are enabled to regulate the blood-supply to every part, in 
obedience to the nervous system. 

The blood exercises a certain pressure on the walls of all 
parts of the vascular system, which is greatest in the heart it- 
self, high in the arteries, lower in the capillaries, and lowest in 
the veins, in the largest of which it may be less than the atmos- 
pheric pressure, or negative. The heart in the mammal consists 
of four perfectly separated chamibers, each upper and each 
lower pair working synchronously, intermixture of arterial 
and venous blood being prevented by septa and interference in 
working by valves. The heart is a force-pump chiefly, but, to 
some extent, a suction-pump also, though its power as such 
purely from its own action and independent of the respiratory 
movements of the chest is slight under ordinary circumstances. 
In consequence of the lesser resistance in the pulmonary divis- 
ion of the circulation, the blood-pressure within the heart is 
much less in the right than in the left ventricle — a fact in har- 
mony with and causative of the greater thickness of the walls 
of the latter ; for in the foetus, in which the conditions are dif- 
ferent, this distinction does not hold. 

The ventricles usually completely empty themselves of 
blood and maintain their systolic contraction even after this 
has been effected. The contraction of the heart, which really 
begins in the great veins near their junction with the auricles 
(that do not fully empty themselves), is at once followed up by 
the auricular and ventricular contraction, the whole constitu- 
ting one long peristaltic wave. Then follows the cardiac pause, 
which is of longer duration than the entire systole. 

When the heart contracts it hardens, owing to closing on a 
non-compressible fluid dammed back within its walls by resist- 
ance a fronte. At the same time the hand placed on the chest- 
walls, over the heart is sensible of the cardiac impulse, owing 
to what has just been mentioned. The systole of the chambers 
of the heart gives rise to a first and a second sound, so called, 


caused by several events combined, in which, however, the ten- 
sion of the valves must take a prominent share. The work of 
the heart is dependent on the quantity of blood it ejects and 
the pressure against which it acts. The pulse is an elevation 
of the arterial wall, occurring with each heart-beat, in conse- 
quence of the passage of a wave over the general blood-stream. 
There is a distention of the entire arterial system in every di- 
rection. The pulse travels with extreme velocity as compared 
with the blood-current. The heart-beat varies in force, fre- 
quency, duration, etc., and with age, sex, posture, and numer- 
ous other circumstances. 

The whole of the circulatory system is regulated by the cen- 
tral nervous system through nerves. There is in the medulla 
oblongata a small collection of nerve-cells making up the 
cardio-inhibitory center. This center, with varying degrees of 
constancy, depending on the group of animals and the needs 
of the organism, sends forth impulses (which modify the beat 
of the heart in force and frequency) through the vagi nerves. 
There are nerves of the sympathetic system with a center in 
the cervical spinal cord, and possibly another in the medulla, 
which are capable of originating either an acceleration of the 
heart-rhythm or an increase of the force of the beat, or both 
together, known as accelerators or augmentors. In the verte- 
brates thus far examined the vagus is in reality a vago-sympa- 
thetic nerve, containing inhibitory fibers proper, and sympa- 
thetic, accelerator, or motor fibers. 

The inhibitory fibers can arrest, slow, or weaken the cardiac 
beat; the sympathetic accelerate it or augment its force. 
When both are stimulated together, the inhibitory prevail. 

These _^nerves, as also the accelerators, exercise a profound 
influence upon the nutrition of the heart, and effect its electri- 
cal condition when stimulated, and we may believe when influ- 
enced by their own centers. 

The inhibitory fibers tend to preserve and restore cardiac 
energy ; the sympathetic, whether in the vagus or as the aug- 
mentors, the reverse. The vagus nerve (and probably the de- 
pressor) acts as an afferent, cardiac sensory nerve reporting on 
the intra - cardiac pressure, etc., and so enabling the vaso- 
motor and cardio-inhibitory centers, which are, it would seem, 
capable of related and harmonious action to act for the general 

The arterioles must be conceived as undergoing very fre- 
quent changes of caliber. They are governed by the vaso- 


motor center, situated in the medulla, and possibly certain sub- 
ordinate centers in the spinal cord, through vaso-motor nerves. 
These are (a) vaso-constrictors, which maintain a constant but 
variable degree of contraction of the muscle-cells of the vessels ; 
(ft) vaso-dilators, which are not in constant functional activity ; 
and (c) mixed nerves, with both kinds. An inherited tendency 
to rhythmical contraction throughout the entire vascular sys- 
tem, including the vessels, must be taken into account. 

The depressor nerve acts by lessening the tonic contraction 
of (dilating) the vessels of the splanchnic area especially. 

It is important to remember that all the changes of the 
vascular system, so long as the nervous system is intact — i. e., 
so long as an animal is normal — are correlated ; and that the 
action of such nerves as the depressor is to be taken rather as 
an example of how some of these changes are brought about, 
mere chapters in an incomplete but voluminous history, if we 
could but write it all. The changes in blood-pressure, by the 
addition or removal of a considerable quantity of blood, are 
slight, owing to the sort of adaptation referred to above, effected 
through the nervous system. Finally, the capillary circulation, 
when studied microscopically, and especially in disordered con- 
ditions, shows clearly that the vital properties of these vessels 
have an important share in determining the character of the 
circulation in themselves directly and elsewhere indirectly. 

The study of the circulation in other groups shows that 
below birds the arterial and venous blood undergoes mixture 
somewhere, usually in the heart, but that in all the vertebrates 
the best blood is invariably that which passes to the head and 
upper regions of the body. The deficiencies in the heart, owing 
to the imperfections of valves, septa, etc., are in part counter- 
acted in some groups by pressure relations, the blood always 
flowing in the direction of least resistance, so that the above- 
mentioned result is achieved. 

Capillaries are wanting in most of the invertebrates, the 
blood flowing from the arteries into spaces (sinuses) in the tis- 
sues. It is to be noted that a modified blood (lymph) is also 
found in the interspaces of the cells of organs. Indeed, the 
circulatory system of lower forms is in many respects analogous 
to the lymphatic system of higher ones. 




The processes of digestion may be considered as having 
for their end the preparation of food for entrance into the 

This is in part attained when the insoluble parts have been 
rendered soluble. At this stage it becomes necessary to inquire 
as to what constitutes food or a food. 

Inasmuch as animals, unlike plants, derive none of their 
food from the atmosphere, it is manifest that what they take in 
by the mouth must contain every chemical element, in some 
form, that enters into the composition of the body. 

But actual experience demonstrates that the food of animals 
must, if we except certain salts, be in organized form — i. e., it 
must approximate to the condition of the tissues of the body in 
a large degree. Plants, in fact, are necessary to animals in 
working up the elements of the earth and air into form suit- 
able for them. 

Foodstuffs are divisible into : 

I. Organic. 

1. Nitrogenous. 

(a.) Albumins. 

(&.) Albuminoids (as gelatine). 

2. Non-nitrogenous. » 

(o.) Carbohydrates (sugars, starches). 
(•6.) Fats. 

II. Inorganic. 

1. Water. 

2. Salts. 

Animals may derive the whole of their food from the 
bodies of other animals {carnivore) ; from vegetable matter 
exclusively (herbivora) ; or from a mixture of the animal and 
vegetable, as in the case of the pig, bear, and man himself 

It has been found by feeding experiments, carried out mostly 
on dogs, that animals die when they lack any one of the con- 
stituents of food, though they live longer on the nitrogenous 
than any other kind. In some instances, as when fed on gela- 
tine and water, or sugar and water, the animals died almost as 
soon as if they had been wholly deprived of food. But it has 
also been observed that some animals will all but starve rather 
than eat certain kinds of food, though chemically sufficient. 



We must thus recognize sometlimg more in an animal than 
merely the mechanical and chemical processes which sufBce to 
accomplish digestion in the laboratory. A food must be not 
only sulficient from the chemical and physical point of view, 
but be capable of being acted on by the digestive juices, and 
of such a nature as to suit the particular animal that eats it. 

To illustrate, bones may be masticated and readily digested 
by a hyena, but not by an ox or by man, though they meet the 
conditions of a food in containing all the requisite constituents. 
Further, the food that one man digests readily is scarcely digesti- 
ble at all by another ; and it is within the experience of every 
one that a frequent change of diet is absolutely necessary. 

Since all mammals, for a considerable period of their exist- 
ence, feed upon milk exclusively, this must represent a perfect 
or typical food. It will be worth while to examine the compo- 
sition of milk. The various substances composing it, and their 
relative proportions for different animals, may be seen from the 
following table, which is based on a total of 1,000 parts : 


















( 48-28 

) 5-76 





I 20-18 

[ 57-03 


Total solids 




• 89-76 

The fact that human milk is poorer in proteids and fats 
especially is of practical importance, for, when cow's milk is sub- 
stituted in the feeding of infants, it should be diluted, and sugar 
and cream added if the normal proportions of mother's milk 
are to be retained. 

1. The proteids of milk are : 

(a.) An albumin very like serum-albumin. 

(6.) Casein, normally in suspension, in the form of extremely 
minute particles, which contributes to the opacity of milk. 

It can be removed by filtration through porcelain ; and pre- 
cipitated or coagulated by acids and by rennet, an extract of 
the mucous membrane of the calf's stomach. After this coagu- 
lation, whey, a fluid more or less clear, separates, which con- 
tains the salts and sugar of milk and most of the water. Much 
of the fat is entangled with the casein. 


Casein, with some fat, makes up the greater part of cheeseV 

2. Fats. — Milk is an enmlsion — i. e., contains fat suspended 
in a fine state of division. The globules, which vary greatly in 
size, are surrounded by an envelope of proteid matter. This 
covering is broken up by churning, allowing the fatty globules 
to run together and form butter. 

Butter consists chiefly of olein, palmitin, and stearin, but 
contains in smaller quantity a variety of other fats. The ran- 
cidity of butter is due to the presence of free fatty acids, espe- 
cially butyric. 

The fat of milk usually rises to the surface as cream when 
milk is allowed to stand. 

3. Milk-sugar, which is converted into lactic acid, probably 
by the agency of some form of micro-organism, thus furnish- 
ing acid sufficient to cause the precipitation or coagulation of 
the casein. 

Milk-sugar. Lactic acid. 

CeH^Oj = aCsHjOj 

Milk, when fresh, should be neutral or faintly alkaline. 

4. Salts (and other extractives), consisting of pliosphates of 
calcium, potassium, and magnesium, potassium chloride, with 
traces of iron and other substances. 

It can be readily understood why children fed on milk rarely 
suffer from that deficiency of calcium salts in the bones leading 
to rickets, so common in ill-fed children. It thus appears that 
milk contains all the constituents requisite for the building up 
of the healthy mammalian body ; and experiments prove that 
these exist in proper proportions and in a readily digestible 
form. The author has found that a large number of animals, 
into the usual food of which, in the adult form, milk does not 
enier, like most of our wild mammals, as well as most birds, 
will not only take milk but soon learn to like it, and thrive well 
upon it. Since the embryo chick lives upon the egg, it might 
have been supposed that eggs would form excellent food for 
adult animals, and common experience proves this to be the 
case ; while chemical analysis shows that they, like milk, con- 
tain all the necessary food constituents. Meat (muscle, with 
fat chiefly) is also, of course, a valuable food, abounding in 
proteids. Cereals contain starch in large proportion, but also a 
mixture of proteids. Green vegetables contain little actual nu- 
tritive material, but are useful in furnishing salts and special 
substances, as certain compounds of sulphur which, in some ill- 
understood way, act beneficially on the metabolism of the body. 



They also seem to stimulate the flov of healthy digestive fluids. 
Qondiments act chiefly, perhaps, in the latter way. Tea, coffee, 
etc:, contain alkaloids, which it is likely have a conservative 
effect on tissue waste, but we really know very little as to how 
it is that they prove so beneficial. Though they are recognized 
to have a powerful effect on the nervous system as stimulants, 
nevertheless it would be erroneous to suppose that their action 
was confined to this alone. 

Animal Foods. 

Explanation of the signs. 



Human mllH. 

JProteids. Albumlnoide. N-/re6 org. bodies. Salts. 








Vegetable Foods. 

^Explanation of the signs. 






White Turnip. 




13 g 

Digestible Non-digestible 
N-free organ bodies. 




Fig. 248 (Landois). 


1 1.4 
I 2.5 

1 1.5 





The accompanying diagrams will serve to represent . to the 
eye the relative proportions of the food-essentials in various 
kinds of articles of diet. 


It is plain that if, in the digestive tract, foods are fchanged 
in solubility and actual chemical constitution, this nmst have 

Fib. 249.— Alimentary canal of embryo while the rudimentary mid-gut is still in eontinui^ 
with yelk-sac (Kolliker, after Bischoff). A. View from before, a, pharyngeal plates ; 6, 
pharynx ; c, c, diverticula forming the lun^ ; d, stomach ; /, aiverticula of liver ; £f, 
membrane torn from yelk-sac ; h, hind-gut. B. Longitudinal section, a, diverticulum of 
a lung ; b, stomach ; c, liver ; d, yelk-sac. 

been brought about by chemical agencies. That food is broken 
up at the very commencement of the alimentary tract is a 
matter of common observation; and that there should be a 
gradual movement of the food from one part of the canal to 

Fig. 250. 

Fio. 251. 

Fig. 250.— Diagram of alimentary canal of chick at fourth day (Foster and Balfour, after 
Gotte). iff, diverticulum of one lung ; St^ stomach ; Z, liver ; p, pancreas. 

Fig. 251.— Position of various parts of alimentary canal at different stages. A. Embryo of 
five weeks. B. Of eight weeks. C. Of ten weeks (Allen Thompson), t, pharynx with the 
lungs; 5, stomach ;, I, small intestine; i\ large intestine; g. genital duc^; u. bladder; 
c2. cloaca ; c, csecum ; vi, ductus vitello-intestinalis ; «i, iu:ogenital sinus ; v. yelk-sae. 



another, where a different fluid is secreted, -would be expected. 
As a matter of fact, mechanical and chemical forces play a 
large part in the actual preparation of the food for absorption. 
Behind these lie, of course, the vital properties of the glands, 
which prepare the active fluids from the blood, so that a study 
of digestion naturally divides itself into the consideration of— 

Pig. f!S!i.—Ammothea pyenogonides, a marine animal (after Quatretages). as, cesophagus: 
a. antennae ; «, stomach, with prolongations into antennse and limbs (0. 

1. The digestive juices ; 2. The secretory processes ; and, 3. The 
muscular and nervous mechanism by which the food is carried 
from one part of the digestive tract to another, and the waste 
matter finally expelled. 

Fio. 353. — Longitudinal vertical section of body of leech. Hirudo medicinalis (after Leuckart). 
a. mouth ; b, b, 6, sacculations of alimentary canal ; c, anus ; d. terminal sucker; e, cere- 
bral ganglia ; /, /', chain of post-oesophageal ganglia ; g^ g, g, segmental organs. 

Embryologlcal. — The alimentary tract, as we have seen, is 
formed by an infolding of the splanchnopleure, and, according 
as the growth is more or less marked, does the canal become 



tortuous or remain somewhat straight. The alimentary tract 
of a mammal passes through stages of development which cor- .„ 
respond with the permanent form of other groups of verte- 
brates, according to a general law of evolution. Inasmuch as 
the embryonic gut is formed of mesoblast and hypoblast, it is 
easy to understand why the developed tract should so invaria- 
bly consist of glandular structures and muscular tissue dis- 
posed in a certain regular arrangement. The fact that all the 

'"■" ■tlrtin!'il\ll',il\tt""- 

Fig. 254. — Portion of a jelly-fish, the Medusa Aurelia^ showing gastro-vascular canals radi- 
ating from central stomach and terminating in a circular marginal canal (after Romanes). 
AU'these are shaded very dark ; the light spaces indicate artificial sections. Inasmuch as 
these canals as well as the stomach must contain some searwater, and since their contents 
represent the whole of the nutritive fluid (answering to the blood, lymph, and chyle of 
higher forms), we have both anatomically and physiologically a very crude or undifferen- 
tiated condition in such animals, and one of great interest from an evolutionary point of 

organs that pour digestive juices into the alimentary tract are 
outgrowths from it serves to explain why there should remain 
a physiological connection with an anatomical isolation. The 
general resemblance of the epithelium throughout, even in 
parts widely separated, also becomes clear, as well as many 
other points we can not now refer to in detail, to one who 
realizes the significance of the laws of descent (evolution). 

Comparative.^Amoeba ingests and digests apparently by 
every part of its body ; though exact studies have shown that 
it neither accepts nor. retains without considerable power of 



discrimination ; and it is also possible that some sort of digest- 
ive fluid may be secreted from the part of the body with which 
the food-particles come in contact. It has been shown, too, 
that there are differences in the digestive capacity of closely 
allied forms among Infusorians. 

The ciliated Inftisorians have a permanent mouth, which 
may also serve as an anus ; or, there may be an anus, though 
usually less distinct from the rest of the body than the mouth. 

Among the Ccslenterates inira-cellular digestion is found. 
Certain cells of the endoderm (as in Hydra) take up food-parti- 

FiG. 855.— A JeJly-flsh, the Medusa Ldmnocodium (after AUman). Note the long proboscis 
(mouth) leading up to the stomach, from which radiate the gastro-vascular canals. A 
portion of the bell has been removed, showing the generative arranged around the 
digestive organs. Most of the. tentacles are turned up. 

cles Amoeba-like, digest them, and thus provide material for 
other cells as well as themselves, in a form suitable for assimi- 
lation. This is a beginning of that differentiation of function 



which is carried so far among the higher vertebrates. But, as 
recent investigations have shown, such intra-cellular digestion 
exists to some extent in the alimentary canal of the highest 
members of the vertebrate group (see page 345). 

The means for grasping and triturating food among in- 
vertebrates are very complicated and varied, as are also those 
adapted for sucking the juices of prey. Examples to hand are 
to be found in the crab, crayfish, spider, grasshopper, beetle. 

Fig. 256. — ^Diagram illustrating arrangement of intestine, nervous system, etc., in common 
snail, Helix (after Huxley), m, mouth ; *, tooUi ; od, odontophore ; g, gullet ; c, crop ; 
s, stomach ; r, rectum ; a, anus ; r. a, renal sac ; h, heart ; Z, lung (modified pallial cham- 
ber) ; n, its external aperture ; em, thick edge of mantle united with sides of body ; /, 
foot ; cpg, cerebral, pedal, and parieto-splancnnic ganglia aggregated round gullet. 

etc., on the one hand, and the butterfly, house-fly, leech, etc., 
on the other. 

The digestive system of the earth-worm has been studied 
with some care. It illustrates a sort of extra-corporeal diges- 
tion, in that it secretes a fluid from the mouth which seems to 
act both chemically and mechanically on the starch-grains 
of the leaves on which it feeds. It is provided with an organ 
in which, as with birds, small stones are found, so that- the 
imperfections of its mouth are compensated for by this gizzard 
which triturates the food. Its calciferous glands supply the 
alkaline fluids necessary to neutralize the humus acids of de- 
caying leaves, for intestinal digestion only proceeds in an alka- 
line medium. 

The gastric mill of a crab (Fig. 228) is a provision of ob- 
vious value in so voracious a creature. 



Before passing on to higher groups, it will be well to bear 
in mind that the digestive organs are to be regarded as the out- 
come both of he- 

Si >ito.A "■ifcTWl •- 

c— ^ P o H^ e 

*• s o & 3 aj-o 

redity and adap- 
tation to circum- 
stances. We find 
parts of the in- 
testine, e. g., re- 
tained in some 
animals in whose 
economy they 
seem to serve 
little if any good 
purpose, as the 
vermiform ap- 
pendix of man. 
Adaptation has 
been illustrated 
in the lifetime 
of a single indi- 
vidual in a re- 
markable man- 
ner ; thus, a sea- 
gull, by being fed 
on grain, has had 
its stomach, nat- 
urally thin and 
soft-wajled, con- 
verted into a 
muscular giz- 

Since diges- 
tion is a process 
in which the 
mechanical and 
chemical are 
both involved, 
and the food of 
animals differs 
so widely, great variety in the alimentary tract, both ana- 
tomical and physiological, must be expected. Vegetable food 
must usually be eaten in much larger bulk to furnish the 
needed elements ; hence the great length of intestine habitually 

Si §1 Shi 

I VI -a? 

0.2 5 

CQ qo p <u 


oiS e8«M gS^ <u*C f 



found in herbivorous animals, associated often with a.capacious 
and chambered stomach, furnishing a larger laboratory in 

Fig. 258. — The viscera of a rabbit as seen upon simply opening the cavities of the thorax and. 
abdomen without any further dissection. A, cavity of the thorax, pleural cavity on either 
side ; .B, diaphragm ; C, ventricles of the heart ; P, auricles ; E^ pulmonary artery; ^^ 
aorta ; ff, lungs collapsed, and occupying only back part of chest ; H, lateral portions of 
pleural membranes ; Z, cartilage at the end of sternum (ensiform cartilage) ; K^ portion 
of the wall of body left between thorax and abdomen ; a, cut ends of the ribs ; L, (he 
liver, in this case lying more to the left than to the right of the body ; M, the stomach, a 
large part of the greater curvatm-e being shown ; 'N, duodenum ; O, small intestine ; P, 
the caecum, so largely developed in this and other herbivorous animals ; C the large 
intestine. (Huxley.) 

which Nature may carry on her processes. To illustrate, the 
stomach of the ruminants consists of four parts (rwmen, reticu- 
lum, omasum ( psalterium,) , abomaswm) . The food when cropped 
is immediately swallowed ; so that the paunch {rumen) is a- 
mere storehouse in which it is softened, though but little 
changed otherwise ; and it would seem that real gastric di- 



gestion is almost confined to tlie last division, which may be 
compared to the simple stomach of the Carnivora or of man ; 
and, before the food reaches this region, it has been thoroughly 
masticated and mixed with saliva. 

Fig. 259.— stomach, pancreas, large intestine, etc. (after Sappey). 1, anterior surface of 
liver ; 3, gall-bladder ; 3, 3, section of diaphragm ; 4, posterior surface of stomach ; 5, 
lobus Spigelii of liver ; 6, coeliac axis ; 7, coronary artery of stomach ; 8, splenic artery ; 
9, spleen; 10, pancreas; 11, superior mesenteric vessels; 12. duodenum; 13, upper ex- 
tremity of small intestine ; 14, lower end of ileum ; 1.5, 15, mesentery ; 16, ceecum ; 17, 
appendix vermiformis ; 18, ascending colon ; 19, 19, transverse colon ; 20, descending 
colon ; 21, sigmoid flexure of colon ; 22, rectum ; 23, urinary bladder. 

The reticulum is especially adapted for holding water, which 
may serve a good purpose in moistening and thinning the con- 
tents of the stomach. In the camels and llamas a portion of 
the stomach is made up of pouches, which can be closed with 
sphincter muscles, and thus shut off the water-supply in sep- 
arate tanks, as it were. 

The stomach of the horse is small, thorfgh the intestine, 
especially the large gut, is capacious. 

The stomach is divisible into a cardiac region, of a light 
color internally, and lined with epithelium, like that of the 


A -«1 

FiQ. 260.— A. Stomach of sheep. B. Stomach of musk-deer, ce, oesophagus ; Rn, rumen ; 
Bet, reticulum ; Ps, psalterium ; ^, ^6, abomeisum ; i>u, duodenum ; Ptf, pylorus (Huxley). 

Fio. 261.— Stomach of horse (after Qiauveau). A, cardiac extremity of oesophagus; B, 

pyloric ring. 



(fesophagus, and a redder pyloric area, in which the greater 
part of the digestive process goes on. 

Fio. 26%.— Stomach of dog (after Chauveau). A, oesophagus ; B, pylorus. 

The mouth parts, even in some of the higher vertebrates, as 
the Carnivora, serve a prehensile rather than a digestive pur- 
pose. This is well seen in the dog, that bolts his food ; but 
in this and allied groups of mammals gastric digestion is very 

Fio. 263.— General and lateral view of dog's teeth (after Chauveau). 



The teeth as triturating organs find their highest develop- 
ment in ruminants, the combined side-to-side and f orward-and- 
backward motion of the jaws rendering them very effective. 

Fig. 265. 

Fig. 364. 

Fig. 264. — Dentition of inferior jaw of horse (after Cliauveau). 

Fig. 265.— Inferior maxilla of man (after Sappey). Alveolar border ; /, incisor teeth : c, canine 
teeth ; b, bicuspid teeth ; m, molars. 

In Carnivora the teeth serve for grasping and tearing, while 
in the Insectivora the tongue, as also in certain birds (wood- 
peckers), is an important organ for securing food. 

It is to be noted, too, that, while the horse crops grass by 
biting it off, the ox uses the tongue, as well as the teeth and 
lips, to secure the mouthful. 



'Fia. 266.— General view of digestive apparatus of fowl (after Chauveauj. I, tongue; 2, 
pharynx : 3, first portion of cesophagus ; 4, crop ; 5, second portion of oesophagus ; 6, 
Buccentric ventricle (proventriculus) ; 7, gizzard ; 8, origin of duodenum ; 9, fct branch 



of duodenal flexure ; 10, second branch of same ; 11, origin of floating portion of small 
intestine ; 12, small intestine ; 12', terminal portion of this intestine, flanked on each side 
hy the two ceeca (regarded as the analogue of colon of mammals) : 13, 13, free extremities 
of csecums ; 14, insertion of these two cuU-de-sac into intestinal tube ; 15, rectum : 16, 
cloaca ; 17, anus ; 18, mesentery ; 19, left lobe of liver ; 20, right lobe ; 21, gall-bladder ; 
22, insertion of pancreatic and biliary ducts ; the two pancreatic ducts are the most ante- 
rior, the choledic or hepatic is in the middle, and the cystic duct is posterior ; S3, pancreas ; 
24, diaphragmatic aspect of lung ; 25, ovary (in a state of atrophy) ; 26, oviduct. 

Man's teeth, are somewhat intermediate in form between the 
carnivorous and the herbivorous type. Birds lack teeth, but 
the strong muscular gizzard suffices to grind the food against 
the small pebbles that are habitually swallowed. 

The crop, well developed in granivorous birds, is a dilata- 
tion of the oesophagus, serving to store and soften the food. 

In the pigeon a glandular epithelium in the crop secretes a 
milky-looking substance, that is regurgitated into the mouth 
of the young one, which is inserted within that of the parent 

The proventriculus — an enlargement just above the gizzard 
— is relatively to the latter very thin-walled, but provides the 
true gastric juices. 

Certain plants digest proteid matter, like animals ; thus the 
sun-dew (Drosera), by the closure of its leaves, captures insects, 
which are digested and the products absorbed. The digestive 
fluid consists of a pepsin-containing secretion, together with 
formic acid. 

The Digestive Juices. 

Saliva. — The saliva as found in the mouth is a mixture of 
the secretion of three pairs of glands, alkaline in reaction, of a 
specific gravity of 1002 to 1006, with a small percentage of 
solids ("2 'per cent), consisting of salts and organic bodies 
(mucin, proteids). 

Saliva serves mechanical functions in articulation, in moist- 
ening the food, and dissolving out some of its salts. But its 
principal use in digestion is in reducing starchy matters to a 
soluble form, as sugar. So far as known, the other constituents 
of the food are not changed chemically in the mouth. 

The Amylolytic Action of Saliva. — Starch exists in grains, sur- 
rounded by a cellulose covering, which saliva does not digest ; 
hence its action on raw starch is slow. 

It is found that if a specimen of boiled starcb not too thick 
be exposed to a small quantity of saliva at the temperature of 
the body or thereabout (37° to 40° C), it will speedily undergo 
certain changes : 

1. After a very short time sugar may be detected by Feh- 


ling's solution (copper sulphate in an excess of sodium hydrate, 
the sugar reducing the cupric hydrate to cuprous oxide on 

2. At this early stage starch may still be detected by the 
blue color it gives with iodine ; but later, instead of a blue, a 
purple ,or red may appear, indicating the presence of dextrin, 
which may be regarded as a product intermediate between 
starch and sugar. 

3. The longer the process continues, the more sugar and the 
less starch or dextrin to be detected; but, inasmuch as the 
quantity of sugar at the end of the process does not exactly 
correspond with the original quantity of starch, even when no 
starch or dextrin is to be found, it is believed that other bodies 
are formed. One of these is achroodextrin, which does not give 
a color reaction with iodine. 

The sugars formed are : (a) Dextrose, (b) Maltose, which 
has less reducing power over solutions of copper salts, a more 
pronounced rotatory action on light, etc. 

It is found that the digestive action of saliva, as in the 
above-described experiment, will be retarded or arrested if the 
sugar is allowed to accumulate in large quantity. That diges- 
tion in the mouth is substantially the same as that just de- 
scribed can be easily shown by holding a solution of starch in 
the mouth for a few seconds, and then testing it for sugar, 
when it will be invariably found. 

While salivary digestion is not impossible in a neutral 
medium, it is arrested in an acid one even of no great strength 
(less than one per cent), and goes on best in a feebly alkaline 
medium, which is the condition normally in the mouth. Though 
a temperature about equal to that of the body is best adapted 
for salivary digestion, it will proceed, we have ourselves found, 
at a higher temperature than digestion by any other of the 
juices, so far as man is concerned — a fact to be connected, in all 
probability, with his habit for ages of taking very warm 
fluids into the mouth. 

^ The active principle of saliva is ptyalin, a nitrogenouS/body 
which is assumed to exist, for it has never been perfectly iso- 
lated. It belongs to the class of unorganized ferments, the 
properties of which have been already referred to before (page 

Characteristics of the Secretion of the Different Glands. — Parotid 
saliva is in man not a viscid fluid, but clear and limpid, con- 
taining very little mucin. Submaxillary saliva in most animals 


and in man is viscid, while the secretion of the sublingual 
gland is still more viscid. 

ComparatiTe. — Saliva differs greatly in activity in different 
animals; thus saliva in the dog is almost inert, that of the 
parotid gland quite so ; in the cat it is but little more effective ; 
and in the horse, ox, and sheep, it is known to be of very feeble 
digestive power. 

In man, the Guinea-pig, the rat, the hog, both parotid and 
submaxillary saliva are active; while in the rabbit the sub- 
maxillary saliva, the reverse of the preceding, is almost in- 
active, and the parotid secretion very powerful. 

An aqueous or glycerine extract of the salivary glands has 
digestive properties. The secretion of the different glands 
may be collected by passing tubes or cannulas into their ducts. 

Pathological. — Potassium sulphocyanate (which gives a red 
color with salts of iron) is sometimes present normally, but is 
said to be in excess in certain diseases, as rheumatism. 

The saliva, normally neutral or only faintly acid, may be- 
come very much so in the intervals of digestion The rapid 
decay of the teeth occurring during and after pregnancy 
seems in certain cases to be referable in part to an abnormal 
condition of the saliva, and in part to the drain on the lime 
salts in the construction of the bones of the foetus. 

The tartar which collects on the teeth consists largely of 
earthy phosphates. 

Gastric Juice. — Gastric juice may be obtained from a fistu- 
lous opening into the stomach. Such may be made artificially 
by an incision over the organ in the middle line, catching it up 
and stitching it to the edges of the wound, incising and insert- 
ing a special form of cannula, which may be closed or opened 
at will. 

Digestion in a few cases of accidental gastric fistulse has 
been made the subject of careful study. The most instructive 
case is that of Alexis St. Martin, a French Canadian, into 
whose stomach a considerable opening was made by a gunshot- 

Gastric juice in his case and in the lower animals with arti- 
ficial openings in the stomach, has been obtained by irritating 
the mucous lining mechanically with a foreign body, as a feather. 

The great difficulty in all such cases arises from the impos- 
sibility of being certain that such fluid is normal ; for the con- 
ditions which call forth secretion are certainly such as the 
stomach never experiences in the ordinary course of events. 



and we have seen how saliva varies, according as the animal is 
fasting or feeding, etc. 

Bearing in mind, then, that our knowledge is possibly only 
approximately correct, we may state what is known of the se- 
cretions of the stomach. 

The gastric secretion is clear, colorless, of low specific grav- 
ity (1001 to 1010)5 the solids being in great part made up of pep- 

Fia. 267.— Gastric fistula in case of St. Martin (after Beaumont). A, A, A, B, borders of open- 
ing into stomach ; C, left nipple ; D, chest ; E, cicatrices from wound made for removal 
of a piece of cartilage ; F, F, F, cicatrices of original wound. 

sin with a small quantity of mucus, which may become excess- 
ive in disordered conditions. There has been a good deal of 
dispute as to the acid found in the stomach during digestion. 
It is now generally agreed that during the greater part of the 
digestive jjrocess there is free hydrochloric acid to the extent 
of about "3 per cent. It is maintained that lactic acid exists 
normally in the early stages of digestion, and it is conceded 
that lactic, butyric, acetic, and other acids may be present in 
certain forms of disordered digestion. 

It is also generally acknowledged that in mammals the 
work of the stomach is limited, so far as actual chemical 
changes go, to the conversion of the proteid constituents of 
food into peptone. Fats may be released from their proteid 
coverings (cells), but neither they nor starches are in the least 
altered chemically. Some have thought that in the dog there 
is a slight digestion of fats in the stomach. The solvent 


power of the gastric juice is greater than can be accounted 
for hy the presence of the acid it contains merely, and it has 
a marked antiseptic action. 

Digestive processes may be conducted out of the body in a 
very simple manner, "which the student may carry out for 
himself; To illustrate by the case of gastric digestion: The 
mucous membrane is to be removed from a pig's stomach 
after its surface has been washed clean, but not too thoroughly, 
chopped up fine, and divided into two parts. On one half pour 
water that shall contain "2 per cent hydrochloric acid (made 
by adding 4 to 6 cc. commercial acid to 1,000 cc. water). This 
will extract the pepsin, and may be used as the menstruum in 
which the substance to be digested is placed. The best is fresh 
fibrin whipped from blood recently shed. 

Since the fluid thus prepared will contain traces of peptone 
from the digestion of the mucous membrane, it is in some 
respects better to use a glycerine extract of the same. This is 
made by adding some of the best glycerine to the chopped-up 
mucous membrane of the stomach of a pig, etc., well dried with 
bibulous paper, letting the whole stand for eight to ten days, 
filtering through cotton, and then through coarse filter-paper. 
It will be nearly colorless, clear, and powerful, a few drops suf- 
ficing for the work of digesting a little fibrin when added to 
some two per cent hydrochloric acid. 

Digestion goes on best at about 40° C, but will proceed in 
the cold if the tube in which the materials have been placed is 
frequently shaken. It is best to place the test-tube containing 
them in a beaker of water kept at about blood-heat. Soon the 
fibrin begins to swell and also to melt away. 

After fifteen to twenty minutes, if a little of the fluid in the 
tube be removed and filtered, and to the filtrate added carefully 
to neutralization dilute alkali, a precipitate, insoluble in water 
but soluble in excess of alkali (or acid), is thrown down. This 
is in most respects like acid-albumen, but has been called para- 
peptone. The longer digestion proceeds, the less is there of 
this and the more of another substance, peptone, so that the 
former is to be regarded as an intermediate product. Peptone 
is distinguished from albuminous bodies or proteids by — 1. 
Not being coagulable from its aqueous solutions on boiling. 
2. Diffusing more readily through animal membranes. 3. Not 
being precipitated by a number of reagents that usually act 
on proteids. 

In artificial digestion it is noticeable that much more fibrin 


or other proteid matter will he dissolved if it be finely divided 
and frequently shaken up, so that a greater surface is exposed 
to the digestive fluid. 

The exact nature of the process by which proteid is changed 
to peptone is not certainly known. 

Since starch on the addition of water becomes sugar (CsHio 
Ob + HsO = CeHisOe), and since peptones have been formed 
through the action of dilute acid at a high temperature or by 
superheated water alone, it is possible that the digestion of 
both starch and proteids may be a hydration j but we do not 
know that it is such. 

As already explained, milk is curdled by an extract of the 
stomach (rennet) ; and this can take place in the absence of all 
acids or anything else that might be suspected except the real 
cause ; there seems to be no doubt that there is a distinct fer- 
ment which produces the coagulation of milk which results 
from the precipitation of its casein. 

The activity of the gastric juice, and all extracts of the mu- 
cous membrane of the stomach, on proteids, is due to pepsin, a 
nitrogenous body, but not a proteid. 

Like other ferments, the conditions under which it is effect- 
ive are well defined. It will not act in an alkaline medium at 
all, and if kept long in such it is destroyed. In a neutral me- 
dium its power is suspended but not destroyed. Digestion will 
go on, though less perfectly, in the presence of certain other 
acids than hydrochloric. As with all digestive ferments, the 
activity of pepsin is wholly destroyed by boiling. 

When a large quantity of cane-sugar is taken into the 
stomach, an excess of mucus is poured out which converts it, 
presumably by means of a special ferment, into dextrose. 

Bile. — The composition of human bile is stated in the fol- 
lowing table : 

Water 83-90 per cent. 

Bile-salts 611 

Fats and soaps 3 

Cholesterin 0"4 

Lecithin 1'5 

Mucin 1"3 

Ash 0-61 

The color of the bile of man is a rich golden yellow. When 
it contains much mucus, as is the case when it remains long in 
the gall-bladder, it is ropy, though usually clear. Bile may 
contain small quantities of iron, manganese, and copper, the 


latter two especially being absent from all other fluids of the 
body. Sodium chloride is the most abundant salt. Bile must 
be regarded as an excretion as well as a secretion ; the pig- 
ments, copper, manganese, and perhaps the iron and the cho- 
lesterin being of little or no use in the digestive processes, so 
far ^s known. 

The bile-salts are the essential constituents of bile as a 
digestive fluid. In man and many other animals, they con- 
sist of taurocholate and glycocholate of sodium, and may be 
obtained in bundles of needle-shaped crystals radiating from 
a common center. These salts are soluble in water and alco- 
hol, with an alkaline reaction, but insoluble in ether. 

Glycocholic acid may be resolved into cholalic (cholic) acid 
and glycin (glycocoll) ; and taurocholic acid into cholalic acid 
and taurin. Thus : 

Glycocholic acid. Cliolalic acid. Glycin. 

C26H43NO6 + H^O = Cs4H4„05 + C2H5NP2. 

Taurocholic acid. Cholalic acid. Taurin. 

C26H45NSO, + Hs„ = C84H4o06 + CsHtNSOs. 

Glycocoll (glycin) is amido-acetic acid— 

Taurin, amido-isethionic acid, 

CsH4< .^.^ , and may be made artificially 

from isethionic acid. 

It is to be noted that the bile acids both contain nitrogen, 
but that chololic acid does not. The decomposition of the bile 
acids takes place in the alimentary canal, and the glycin and 
taurin are restored to the blood, and are possibly used afresh 
in the construction of the bile acids, though this is not defi- 
nitely known. 

Bile-Figments. — The yellowish-red color of the bile is owing 
to Bilirubin (CieHigN'sOa), which may be separated either as 
an amorphous yellow powder or in tablets and prisms. It is 
soluble in chloroform, insoluble in water, and but partially 
soluble in alcohol and ether. It makes up a large part of 
gall-stones, which contain, besides cholesterin, earthy salts in 

It may be oxidized to Biliverdin (C16H18NSO4), the natural 
green pigment of the bile of the herbivora. When a drop of 
nitric acid, containing nitrous acid, is added to bile, it under- 

Digestion op pood. 313 

goes a series of color changes in a certain tolerably constant 
order, becoming green, greenish-blue, blue, violet, a brick red, 
and finally yellow ; though the green is the most characteristic 
and permanent. Each one of these represents a distinct stage 
of the oxidation of bilirubin, the green answering to biliverdin. 
Such is Gmelin's test for bile-pigments, by which they may be 
detected in urine or other fluids. The absence of proteids in 
bile is to be noted. 

The Digestive Action of Bile.— 1. So far as known, its action 
on proteids is nil. When bile is added to the products of an 
artificial gastric digestion, bile-salts, peptone, pepsin, and para- 
peptone are precipitated and redissolved by excess. 2. It is 
slightly solvent of fats, though an emulsion made with bile is 
very feeble. But it is likely helpful to pancreatic jiiice, or 
more efficient itself when the latter is present. With free fatty 
acids it forms soaps, which themselves help in emulsifying fat. 
3. Membranes wet with bile allow fats to pass more readily ; 
hence it is inferred that bile assists in absorption. 4. When 
bile is not poured out into the alimentary canal the faeces 
become clay -colored and ill-smelling, foul gases being secreted 
in abundance, so that it would seem that bile exercises an anti- 
septic influence. It may limit the quantity of indol formed. 
It is to be understood that these various properties of bile are 
to be traced almost entirely to its salts ; though its alkaline 
reaction is favorable to digestion in the intestines, apart from 
its helpfulness in soap-forming, etc. 5. It is thought by some 
that the bile acts as a stimulant to the intestinal tract, giving 
rise to peristaltic movements, and also, mechanically, as a lubri- 
cant of the faeces. In the opinion of many, an excess of bile 
naturally poured out causes diarrhoea, and it is well known 
that bile given by the mouth acts as a purgative. However, 
we must distinguish between the action of an excess and that 
of the quantity secreted by a healthy individual. The acid of 
the stomach has probably no effect allied to that produced by 
giving acids medicinally, which warns us that too much must 
not be made out of the argument from bilious diarrhoea. 6. As 
before intimated, a great part of the bile must be regarded as 
excrementitious. It looks as though much of the effete haemo- 
globin of the blood and of the cholesterin, which represents 
possibly some of the waste of nervous metabolism, were expelled 
from the body by the bile. The cholalic acid of the faeces is 
derived from the decomposition of the bile acids. Part of their 
mucus must also be referred to the bile, the quantity originally 



present in this fluid depending much on the length of its stay- 
in the gall-bladder, which secretes this substance. 7. There is 
throughout the entire alimentary tract a secretion of mucus 
which must altogether amount to a large quantity, and it has 
been suggested that this has other than lubricating or such like 
functions. It appears that mucus may be resolved into a pro- 
teid and an animal gum, which latter^ it is maintained, like 
vegetable gums, assists emulsification of fats. If this be true, 
and the bile is, as has been asserted, possessed of the power to 
break up this mucus (mucin), its emulsifying effect in the in- 
testine may indirectly be considerable. Bile certainly seems 
to intensify the emulsifying power of the pancreatic juice. 

There does not seem to be any ferment in bile, unless the 
power to change starch into sugar, peculiar to this secretion in 
some animals, is owing to such. 

Comparative. — The bile of the carnivora and omnivora is 
yellowish-red in color; that of herbivora green. The former 
contains taurocholate salts almost exclusively ; in herbivorous 
animals and man there is a mixture of the salts of both acids, 
though the glycocholate predominates. 

Fig. 268.— Gall-bladder, ductus choledochus and pancreas (after Le Bon), a, ^all-bladder ; 
6, hepatic duct ; c, opening of second duct of pancreas ; rf, opening of main pancreatic 
duct and bile-duct ; e, e, duodenum ; /, ductus choledochus ; p, pancreas. 

Pancreatic Juice. — This fluid is foimd to vary a good deal 
quantitatively, according as it is obtained from a temporary 
(freshly made) or permanent fistula — a fact which emphasizes 



the necessity for caution in drawing conclusions about the 
digestive juices as obtained by our present methods. 

The freshest juice obtainable through a recent fistulous 
opening in the pancreatic duct is clear, colorless, viscid, alka- 
line in reaction, and with a very variable quantity of solids 
(two to ten per cent), less than one per cent being inorganic 

Among the organic constituents the principal are albumin, 
alkali-albumin, peptone, leucin, tyrosin, fats, and soaps in small 
amount. The alkalinity of the juice is owing chiefly to sodium 

FiQ. 269.— Crystals of leucin (Funke). 

Fig. 270.— Crystals of tyrosin (Funke). 

carbonates, which seem to be associated with some proteid 
body. There is little doubt that leucin, tyrosin, and peptone 
arise from digestion of the proteids of the juice by its own 

ExperimentaL — If the pancreatic gland be mostly freed from 
adhering fat, cut up, and washed so as to get rid of blood ; 
then minced as fine as possible, and allowed to stand in one-per- 
cent sodium-carbonate solution at a temperature of 40° C, the 
following results may be noted : 1. After a variable time the 
reaction may change to acid, owing to free fatty acid from 
the decomposition (digestion) of neutral fats. 3. Alkali-albu- 
min, or a body closely resembling it, may be detected and sep- 
arated by neutralization. 3. Peptone may be detected by the 
use of a trace of copper sulphate added to a few drops of caustic 
alkali, which becomes red if this body be present. 4. After a 
few hours the smell becomes fgecal, owing in part to indol, 
which gives a violet color with chlorine- water ; while under 
the microsco;^e the digesting mass may be seen to be swarming 


■with bacteria. 5. When digestion has proceeded for some time, 
leucin and tyrosin may be shown to be present, though their 
satisfactory separation in crystalline form involves somewhat 
elaborate details. These changes are owing to self-digestion 
of the gland. 

All the properties of this secretion may be demonstrated 
more satisfactorily by making an aqueous or, better^ glycerine 
extract of the pancreas of an ox, pig, etc., and carrying on arti- 
ficial digestion, as in the case of a peptic digestion, with fibrin. 
In the case of the digestion of fat, the emulsifying power of a 
watery extract of the gland may be shown by shaking up a 
little melted hog's lard, olive-oil (each quite fresh, so as to show 
no acid reaction), or soap. Kept under proper conditions, free 
acid, the result of decomposition of the neutral fats or soap 
into free acid, etc., may be easily shown. The emulsion, though 
allowed to stand long, persists, a fact which is availed of to 
produce more palatable and easily assimilated preparations of 
cod-liver oil, etc., for medicinal use. 

Starch is also converted into sugar with great ease. In 
short, the digestive juice of the pancreas is the most complex 
and complete in its action of the whole series. It is amylolytic, 
proteolytic, and steaptic, and these powers have been attributed 
to three distinct ferments — amylopsin, trypsin, and steapsin. 

Proteid digestion is carried further than by the gastric juice, 
and the quantity of crystalline nitrogenous products formed is 
in inverse proportion to the amount of peptone, from which it 
seems just to infer that part of the original peptone has been 
converted into these bodies, which are found to be abundant or 
not in an artificial digestion, according to the length of time 
it has lasted — the longer it has been under way the more leucin 
and tyrosin present. Leucin is another compound into which 
the amido (NHu) group enters to make amido-caproic acid — one 
of the fatty series — while tyrosin is a very complex member of 
the aromatic series of compounds. Thus complicated are the 
chemical effects of the digestive juices; and it seems highly 
probable that these are only some of the compounds into 
which the proteid is broken up. 

These crystalline bodies may be made artificially by the 
long-continued action under heat of acids and alkalies, in pro- 
teid or gelatinous matter, though it can not be said that these 
facts have as yet thrown much light upon their formation in 
the digestive organs. 

Though putrefactive changes with formation of indol, etc., 


occur in pancreatic digestion, both within and without the 
body, they are to be regarded as accidental, for by proper pre- 
cautions digestion may be carried on in the laboratory without 
their occurrence, and they vary in degree with the animal, the 
individual, the food, and other conditions. It is not, however. 

Fig. 271.— Micro-organisms of large intestine (after Landois). 1, bacterium ooli cummune ; 
2, bacterium lactis aSrogenes ; 3, 4, large bacilli of Bienstock, with partial endogenous 
spore-formation ; 5^ various stages of development of bacillus which causes fermentation 
of albumen. 

to be inferred that micro-organisms serve no useful purpose 
in the alimentary canal ; the subject, in fact, requires further 

Succus Entericus. — The difficulties of collecting the secretions 
of Lieberkiihn's, Briinner's, and other intestinal glands will be 
at once apparent. But by dividing the intestine in two places, 
so as tp isolate a loop of the gut, joining the sundered ends by 
ligatures, thus making the continuity of the main gut as com- 
plete as before, closing one end of the isolated loop, and bring- 
ing the other to the exterior, as a fistulous opening, the secre- 
tions could be collected, food introduced, etc. 

But it seems highly improbable that information approxi- 
mately correct at best, and possibly highly, misleading, could 
be obtained in such manner. Moreover, the greatest diversity 
of opinion prevails as to the facts themselves, so that it seems 
scarcely worth while to state the contradictory conclusions 
arrived at. 

It is, however, on the face of it, probable that the intestine 
— even the large intestine^— does secrete juices, that in herbiv- 
ora, at all events, play no unimportant part in the digestion 
of their bulky food; and it is also probable, as in so many 
other instances, that, when the other parts of the digestive 
tract fail, when the usual secretions are not prepared or do not 
act on the food, glands that normally play a possibly insig- 
nificant part may function excessively — we may almost say 
vicariously — and that such glands must be sought in the small 
intestine. There are facts in clinical medicine that seem to 



X"' ^'^v"^ 


Fig. 272.— General view of horse's intestines ; animal is placed on its back, and intestinal 
mass spread out (after Chaiiveau). A, duodenum as it passes behind great mesenteric 
artery ; B, free portion of small intestine : C, ileocsecal portion ; D, caecum ; E, F, G, 
loop formed by large colon ; G, pelvic'flexure ; F, F, point where colic loop is doubled to 
constitute suprasternal and diaphragmatic flexures. 


point strongly in this direction, thongh the subject has not yet 
been reduced to scientific form. 

Comparative. — Within the last few years the study of vege- 
table assimilation from the comparative aspect has been fruit- 
ful in results which, together with many other facts of vege- 
table metabolism, show that even plants ranking high in the 
organic plane are not in many of their functions so different 
from animals as has been supposed. It has been known for a 
longer period that certain plants are carnivorous ; but it was 
somewhat of a surprise to find, as has been done within the 
past few years, that digestive ferments are widely distributed 
in the vegetable kingdom and are found in many different 
parts of plants. What purpose they may serve in the vege- 
table economy is as yet not well known. At present it would 
seem as though, from their presence in so many cases in the 
seed, they might have something to do with changing the 
cruder forms of nutriment into such as are better adapted for 
the nourishment of the embryo. 

Thus far, then, not only diastase but pepsin, a body with 
action similar to trypsin, and a rennet ferment, rank amo^g 
the vegetable ferments best known. 

A ferment has been extracted from the stem, leaves, and un- 
ripe fruit of Carica papaya, found in the East and West Indies 
and elsewhere, which has a marked proteolytic action. 

It is effective in a neutral, most so in an alkaline medium ; 
and, though its action is suspended in a feeble acid menstruum, 
it does not appear to be destroyed under such circumstances, as 
is trypsin. This body is attracting a good deal of attention, 
and its use has been recently introduced into medical practice. 

Very lately also a vegetable rennet has been found in sev- 
eral species of plants. The subject is highly promising and 

Seckbtion as a Physiological Process. 

Secretion of the Salivary Glands. — We shall treat this subject 
at more length because of the light it throws on the nervous 
phenomena of vital process ; and, since the salivary glands have 
been studied more thoroughly and successfully than any other, 
they will receive greater attention. 

The main facts, ascertained experimentally and otherwise, 
are the following : 

Assuming that the student is familiar with the general ana- 



tomical relations of tlie salivary glands in some mammal, we 
would further remind him that the submaxillary gland has a 
double nervous supply : 1. From the cervical sympathetic by 
branches passing to the gland along its arteries. 2. From the 
chorda tympani nerve, which after leaving the facial makes 
connection with the lingual, whence it proceeds to its destina- 

Part of train above medulla 

Afferent nerves 
from tongue 

'aUvary gland 

Blood vessel 
of gland 

SympatheUe nerve 

Fig. 273. — Diagram intended to indicate the nervous mechanism of salivary secretion. 

The following facts are of importance as a basis for conclu- 
sions : 1. It is a matter of common observation that a flow of 
saliva may be excited by the smell, taste, sight, or even thought 
of food. 2. It is also a matter of experience that emotions, as 
fear, anxiety, etc., may parch the mouth — i. e., arrest the flow of 
saliva. The excited speaker thus suffers in his early efforts. 


3. If a glass tube be placed in the duct of the gland and any 
substance that naturally causes a flow of saliva be placed on 
the tongue, saliva may be seen to rise rapidly in the tube. 4. 
The same may be observed if the lingual nerve, the glossopha- 
ryngeal, and many other nerves be stimulated ; also if food be 
introduced into the stomach through a fistula. 5. If the pe- 
ripheral end of the chorda tympani be stimulated, two results 
follow : (a) There is an abundant flow of saliva, and (b) the 
arterioles of the gland become dilated; the blood may pass 
through with such rapidity that the venous blood may be 
'bright red in color and there may be a venous pulse. 7. Stimu- 
lation of the medulla oblongata gives rise to a flow of saliva, 
which is not possible when the nerves of the gland, especially 
the chorda tympani, are divided ; nor can a flow be then excited 
by any sort of nervous stimulation, excepting that of the ter- 
minal branches of the nerves of the gland itself. 8. If the sym- 
pathetic nerves of the gland be divided, there is no immediate 
flow of saliva, though there may be some dilatation of its ves- 
sels. 9. Stimulation of the terminal ends of the sympathetic 
and chorda nerves causes a flow of saliva, differing as to total 
quantity and the amount of contained solids ; but the nerve 
that produces the more abundant watery secretion, or the re- 
verse, varies with the animal, e. g., in the cat chorda saliva is 
more viscid, in the dog less so ; though in all animals as yet 
examined it seems that the secretion as a result of stimulation 
of the chorda tympani nerve is the more abundant ; and in the 
case of stimulation of the chorda the vessels of the gland are 
dilated, while in the case of the sympathetic they are con- 
stricted. 10. If atropin be injected into the blood, it is impos- 
sible to induce salivary secretion by any form of stimulation, 
though excitation of the chorda nerve still causes arterial dila- 

Conclusions. — 1. There is a center in the medulla presiding 
over salivary secretion. 2. The influence of this center is 
rendered effective through the chorda tympani nerve at all 
events, if not also by the sympathetic. 3. The chorda tym- 
pani nerve contains both secretory and vaso-dilator fibers ; the 
sympathetic secretory and vaso-constrictor fibers. 4. Arterial 
change is not essential to secretion, though doubtless it usually 
accompanies it. Secretion may be induced in the glands of 
an animal after decapitation by stimulation of its chorda 
tympani nerve, analogous to the secretion of sweat in the foot 
of a recently dead animal, under stimulation of the sciatic 



nerve. 5. The character of the saliva secreted varies with 
the nerve stimulated, so that it seems likely that the nervous 
centers normally in the intact animal regulate the quality of 
the saliva through the degree to which one or the other kind 
of nerves is called into action. 6. Secretion of saliva may 
be induced reflexly by experiment, and such is probably the 
normal course of events. 7. The action of the medullary center 
may be inhibited by the cerebrum (emotions). 

Some have located a center in the cerebral cortex (taste cen- 
ter), to which it is assumed impulses first travel from the 
tongue and which then rouses the proper secreting centers in 
the medulla into activity. It seems more likely that the corti- 
cal center, if there be one, completes the physiological processes 
by which taste sensations are elaborated. 

From the influence of drugs (atropin and its antagonist 
pilocarpin) it is plain that the gland can be affected through 
the blood, though whether wholly by direct action on the cen- 
ter, on any local nervous mechanism or directly on the cells, is 
as yet undetermined. It is found that pilocarpin can act long 
after section of the nerves. This does not, however, prove that 
in the intact animal such is the usual modus operandi of this 
or other drugs, any more than the so-called paralytic secretion 
after the section of nerves proves that the latter are not con- 
cerned in secretion. 

We look upon paralytic secretion as the work of the cells 
when gone wrong— passed from under the dominion of the 
nerve-centers. Secretion is a part of the natural life-processes 
of gland-cells — we may say a series in the long chain of pro- 
cesses which are indispensable for the health of these cells. 
They must be either secreting cells, or have no place in the nat- 
ural order of things. It is to be especially noted that the secre- 
tion of saliva continues when the pressure in the ducts of the 
gland is greater than that of the blood in its vessels or even 
of the carotid ; so that it seems possible that over-importance 
has been attached to blood - pressure in secretory processes 

It may, then, be safely assumed that formation of saliva re- 
sults in consequence of the natural activity of certain cells, the 
processes of which are correlated and harmonized by the nerv- 
ous system ; their activity being accompanied by an abundant 
supply of blood. The actual outpouring of saliva depends usu- 
ally on the establishment of a nervous reflex arc. The other 
glands have been less carefully studied, but the parotid is 


known to have a double nervous supply from the cerebro- 
spinal and the sympathetic systems. 

It would appear that, as the vaso-motor changes run paral- 
lel with the secretory ones, the vaso-motor and the proper 
secretory centers act in concert, as we have seen holds of the 
former and the respiratory center. But it is to our own mind 
very doubtful whether the doctrine of so sharp a demarkation 
of independent centers, prominently recognized in the physi- 
ology of the day, will be that ultimately accepted. 

Secretion by the Stomach. — The mucous membrane of St. Mar- 
tin's stomach was observed to be pale in the intervals of diges- 
tion, but flushed when secreting, which resembled sweating, so 
far as the flow of the fluid is concerned. When the man was 
irritated, the gastric membrane became pale, and secretion was 
lessened or arrested, and it is a common experience that emo- 
tions may help, hinder, or even render aberrant the digestive 

While the evidence is thus clear that gastric secretion is 
regulated by the nervous system, the way in which this is 
accomplished is very obscure. We know little of either the 
centers or nerves concerned, and what we do know helps but 
doubtfully to an understanding of the matter, if, indeed, it 
does not actually confuse and puzzle. 

Digestion can proceed in a fashion after section of the nerves 
going to the stomach, though this has little force as an argu- 
ment against nerve influence. We may conclude the subject 
by stating that, while the influence of the nervous system over 
gastric secretion is undoubted as a fact, the method is not 
understood ; and the same remark applies to the secreting 
activity of the liver and pancreas. 

The Secretion of Bile and Pancreatic Juice. — When the contents 
of the stomach have reached the orifice of the discharging bile- 
duct, a large flow of the biliary secretion takes place, probably 
as the result of the emptying of the gall-bladder by the con- 
traction of its walls and those of its ducts. This is probably 
a reflex act, and the augmented flow of bile when digestion is 
proceeding is also to be traced chiefly to nervous influences 
reaching the gland, though by what nerves or under the gov- 
ernment of what part of the nervous centers is unknown. 
Very similar statements apply to the secretion of the pancre- 
atic glands, though this is not constant, as in the case of bile — 
at all events, in most animals. 

It is known that after food has been taken there is a sudden 



increase in the quantity of bile secreted, followed by a sudden 
diminution, then a more gradual rise, with a subsequent fall. 
Almost the same holds for the pancreas. 

Fig. 874.— Diagram to show influence of food in secretion of pancreatic juice (after N. O. Bern- 
stein). The abscissae represent hours after taking food ; ordinates amount in cubic centi- 
grammes of secretion in ten minutes. Food was taken at B and C. This diagram very 
nearly also represents the secretion of bile. 

It seems impossible to explain these facts, especially the 
first rapid discharge of fluid apart from the direct influence of 
the nervous system. 

Upon the whole, the evidence seems to show that the press- 
ure in the bile-ducts is greater than in the veins that unite to 
make up the portal system; but there are difiiculties in the 
investigation of such and kindred subjects as regards the liver, 
owing to its peculiar vascular supply. It will be borne in mind 
that the liver in mammals consists of a mass of blood-vessels, 
between the meshes of which are packed innumerable cells, and 
that around the latter meander the bile capillaries ; that the 
portal vein breaks up into the interlobular, from which capil- 
laries arise, that terminate in the central intralobular veins, 
which make up the hepatic veinlets or terminate in these vessels 
But the structure is complicated by the branches of the hepatic 
artery, which, as arterioles and capillaries, enters to some extent 
into the formation of the lobular vessels. It is remarkable that 
the cells of the liver are so similar, considering the complicated 
functions they appear to discharge. 



A question of interest, thougli difficult to answer, is the 
extent to which the various constituents of bile are manufact- 
ured in the liver. Taurin, for example, is present in some of 

Fig. 275. — Lobules of liver, interlobular vessels, and intralobular veins (Sappey). 1. 1, 1, 1, 3, 4, 
lobules ; 2, 2, 2. 2, intralobular veins in.iected with white ; 5, 5, 5, 5, 5, intralobular vessels 
filled with a dark injection. 

the tissues, but whether this is used in the manufacture of 
taurocholic acid or whether the latter is made entirely anew, 
and possibly by a method 
in which taurin never ap- 
pears as such, is an open 
question. It is highly prob- 
able that a portion of the 
bile poured into the intes- 
tine is absorbed either as 
such or after partial decom- 
position, the products to be 
used in some way in the 
economy and presumably in 
the construction of bile bj^ 
the liver. There are many 
facts, including some patho- 
logical phenomena, that 
point clearly to the forma- 
tion of the pigments of bile 
from haemoglobin in some 
of its stages of degeneration. 

Pathological. — When the liver 

Fig. 276.— Portion of transverse section of hepatic 
lobule of rabbit : magnified 400 diameters 
(Kolliker). b, b. b, capillary blood-vessels ; 
(/, sr, f/, capillary bile-ducts ; /, /, I. liver-cells. 

fails to act either from de- 


rangement of its cells primarily or owing to obstruction to the 
outflow of bile leading to reabsorption by the liver, bile acids 
and bile pigments appear in the urine or may stain the tissues, 
indicating their presence in excess in the blood. 

This action of one gland (kidneys) for another is highly 
suggestive, and especially important to bear in mind in medical 
practice, both in treatment and prognosis. The chances of re- 
covery when only one excreting gland is diseased are much 
greater evidently than when several are involved. Such facts 
as we have cited show, moreover, that there are certain common 
fundamental principles underlying secretion everywhere — a 
statement which will be soon more fully illustrated. 

The Nature of the Act of Secretion. 

We are now about to consider some investigations, more 
particularly their results, which are of extraordinary interest. 

The secreting cells of the salivary, the pancreatic glands, 
and the stomach have been studied by a combination of histo- 
logical and, more strictly, physiological methods, to which we 
shall now refer. Specimens of these glands, both before and 
after prolonged secretion, under stimulation of these nerves, 
were hardened, stained, and sections prepared. As was to be 
expected, the results were not entirely satisfactory under these 
methods; however, the pancreas of a living rabbit has been 
viewed with the microscope in its natural condition; and by 
this plan, especially when supplemented by the more involved 
and artificial method first referred to, results have been reached 

Fig. 277.— Portion of pancreas of rabbit (after Kiihne and Lea). A represents gland at rest ; 

B, during secretion. 

which may be ranked among the greatest triumphs of modern 


Some of these we now proceed to state briefly. To begin 
with, the pancreas, it has been shown that, when the gland is 
not secreting — i. e., not discharging its prepared fluid — or dur- 
ing the so-called resting stage, the appearances are strikingly 
different from what they are during activity. The cell pre- 
sents during rest an inner granular zone and an outer clearer 
zone, which stains more readily, and is relatively small in size. 
The lumen of the alveolus is almost obliterated, and the in- 
dividual cells very indistinct. After a period of secreting 
activity, the lumen is easily perceived, the granules have dis- 
appeared in great part, the cells as a whole are smaller, and 
have a clear appearance throughout. Coincident with the 
changes in the gland's cells it is to be noticed that more blood 
passes through it, owing to dilatation of the arterioles. 

Fig. 278.— Section of mucous gland (after Lavdowsky). In A, gland at rest ; in B, after 
secreting for some time. 

Again, the course of the changes in the salivary glands, 
whether of the mucous or serous variety, is very similar. In 
the mucous gland in the resting stage the cells are large, and 
hold much clear matter in the interspaces of the cell network ; 
and, as this does not stain readily, it can not be ordinary 
protoplasm. This, when the gland is stimulated through its 
nerves, disappears, leaving the containing cells smaller. It 
has become mucin, and may itself be called mucinogen. 

It is to be noted that, as the cells become more protoplasmic, 
less biirdened with the products of their activity, the nucleus 
becomes more prominent, suggestive of its having a probable 
directive influence over these manufacturing processes. 

Substantially the same chain of events has been established 
for the serous salivary glands and the stomach, so that we 
may safely generalize upon these well-established facts. 



It seems clear that a series of changes constructive and, from 
one point of view, destructive, following the former are con- 

FiG. 279.— Changes iu parotid (serous) gland during secretion (after Langley). A, during rest; 
B, after moderate, C, after prolonged stimulation. Figures partly diagrammatic. 

stantly going on in the glands of the digestive organs. Proto- 
plasm Tinder nerve influence constructs a certain substance, 
which is an antecedent of the final product, which we term a 
ferment. It is now customary to speak of these changes as 
constructive (anabolic) and destructive (katabolic), though we 
have already pointed out (page 270) that this view is, at best, 
only one way of looking at the matter, and we doubt if it may 
not be cramping and misleading. 

We must also urge caution in regard to the conception to 
be associated with the use of the terms " resting " and " active " 
stage. It is not to be forgotten that strictly in living cells 
there is no absolute rest — such means death ; but, if these terms 
be understood as denoting but degrees of activity, they need ' 
not mislead. It is also more than probable that in certain of 
the glands, or in some animals, the processes go on simultane- 
ously : the protoplasm being renewed, the zymogen, or mother- 
ferment, being formed, and the latter converted into actual fer- 
ment, all at the same time. 

It has been pointed out that chorda saliva is usually more 
watery than that secreted under stimulation of the sympathetic. 
When atropine is injected there is no discharge whatever, not- 
withstanding that the usual vascular dilatation follows, from 
which it is clear that the water is actually secreted. 

The nature of secretion is now tolerably clear as a whole ; 
though it is to be remembered that this account is but general, 
and that there are many minor differences for each gland and 
variations that can scarcely be denominated minor for different 
animals. Evidently no theory of filtration, no process depend- 
ing solely on blood-pressure, will apply here. And if in this, 
the best-studied case, mechanical theories of vital processes 
utterly fail, why attempt to fasten them upon other glands, as 


the kidneys and the lungs, or, indeed, apply such crude concep- 
tions to the subtle processes of living protoplasm anywhere or 
in any form. ? 

It is somewhat remarkable that an extract of a perfectly 
fresh pancreas is not proteolytic ; yet the gland yields such an 
extract when it has stood some hours or been treated with a 
weak acid. These facts, together with the microscopic appear- 
ances, suggested that there is formed a forerunner to the actual 
ferment — a zymogen, or mother-ferment, which at the moment 
of discharge of the completed secretion is converted into the 
actual ferment. We might, therefore, speak of a pepsinogen, 
typsinogen, etc., and, though there may be a cessation in the 
series of processes, and no doubt there is in some animals, this 
may not be the case in all or in all glands. 

Secretion by the Stomach. — The glands of the stomach differ 
in most animals in the cardiac and pyloric regions. In those 
of the former zone, both central, columnar, and parietal (ovoid) 
cells are to be recognized. It was thought that possibly the 
latter were concerned in the secretion of the acid of the 
stomach, but this is by no means certain. Possibly these, like 
the demilune cells of the pancreas, may be the progenitors of 
the central (chief) cells. The latter certainly secrete pepsin, 
and probably also rennet. Mucus is secreted by the cells lining 
the neck of glands and covering the mucous membrane inter- 
vening between their mouths. The production of hydrochloric 
acid by any act of secretion is not believed in by all writers, 
some holding that it is derived from decomposition of sodium 
chloride, possibly by lactic acid. So simple an origin is not 
probable, not being in keeping with what we know of chemical 
processes within the animal body. 

Self-Digestiou of the Digestive Organs. — It has been found, both 
in man and other mammals, that when death follows in a 
healthy subject while gastric digestion is in active progress 
and the body is kept warm, a part of the stomach itself and 
often adjacent organs are digested, and the question is con- 
stantly being raised. Why does not the stomach digest itself 
during life ? To this it has been answered that the gastric 
juice is constantly being neutralized by the alkaline blood; 
and, again, that the very vitality of a tissue gives it the neces- 
sary resisting powers, a view contradicted by an experiment 
which is conclusive. If the legs of a living frog be allowed to 
hang against the inner walls of the stomach of a mammal 
when gastric digestion is going on, they will be digested. 


The first view (the alkalinity of the blood) would not suffice 
to explain why the pancreas, the secretion of which acts best 
in an alkaline medium, should not be digested. 

It seems to us there is a good deal of misconception about 
the facts of the case. Observation on St. Martin shows that 
the secretion of gastric juice runs parallel with the need of it, 
as dependent on the introduction of food, its quantity, quality, 
etc. Now, there can be little doubt that, if the stomach were 
abundantly bathed when empty with a large quantity of its 
own acid secretion, it would suffer to some extent at least. 
But this is never the case ; the juice is carried off and mixed 
with the food. This food is in constant motion and doubtless 
the inner portions of the cells, which raay be regarded as the 
discharging region, while the outer (next the blood capillaries, 
the chief manufacturing region of the digestive ferment) are 
frequently renewed. 

Such considerations, though they seem to have been some- 
what left out of the case, do not go to the bottom of the 
matter. Amoeba and kindred organisms do not digest them- 
selves. Some believe that the little pulsatile vacuoles of the 
Infusorians are a sort of temporary digestive cavities. 

But, to one who sees in the light of evolution, it must be 
clear that a structure could not have been evolved that would 
be self -destructive. 

The difficulty here is that which lies at the very basis of all 
life. ,We might ask. Why do living things live, since they are 
constantly threatened with destruction from within as from 
without ? Why do not the liver, kidney, and other glands that 
secrete noxious substances, poison themselves ? We can not 
in detail explain these things; but we wish to make it clear 
that the difficulty as regards the stomach is not peculiar to 
that gland, and that even from the ordinary point of view it 
has been exaggerated. 

Comparative. — More careful examination of the stomachs of 
some mammals has revealed the fact that in several animals, 
in which the stomach appears to be simple, it is in reality 
compound. There are different grades, however, which may 
be regarded as transition forms between the true simple 
stomach and that highly compound form of the organ met 
with in the ruminants. 

It has been shown recently that the stomach of the hog has 
an oesophageal dilatation; and that the entire organ may be 
divided into several zones with different kinds of glandular 


epithelium, etc. These portions differ in digestiTe power, in 
the characteristics of the fluid secreted, and other details be- 
yond those which a superficial examination of this organ 
would lead one to suspect. 

The stomach of the horse represents a more advanced form 
of compound stomach than that of the hog, which is not evi- 
dent, however, until its glandular 
structure is examined closely. The 
entire left portion of the stomach 
represents an cesophageal dilata- 
tion lined with an epithelium that 
closely resembles that of the oesoph- 
agus, and with little if any digest- 
ive function. It thus appears that 
the stomach of the horse is in reali- 
ty smaller, as a true digestive gland, 
than it seems, so that a great part 
of the work of digestion must be 
done in the intestine ; though in ^10. 280.-ii,terior of h^s stomach 

+Tni« nnimnl if tVip fnnrl Vip Tpfninprl (after Chauveau). ^, left sac: B, 

tnis anirnai, ir lae rooa oe recamea ^j^j,,. ^^ . ^,_ duodenal dilatation, 
long as it is in the hog, which is 

not, however, the general opinion as regards the stomach of the 
horse, salivary digestion may continue for a considerable period 
after the food has left the mouth. The secretion of mucus by 
the stomach in herbivora is abundant. 

The Movements of the Digestive Organs. 

As with other parts of the body, so in the alimentary tract, 
the slower kind of movement is carried out by plain muscu- 
lar fibers ; and the movements, as a whole, belong to the class 
known as peristaltic ; in fact, it is only at the beginning of the 
digestive tract that voluntary (striped) muscle is to be found 
and to a limited extent in the part next to this — i. e., in the 

Teeth in the highly organized mammal are remarkable in 
being to the least degree living structures of any in the entire 
animal, thus being in marked contrast to other organs. The 
enamel covering their exposed surfaces is the hardest of all the 
tissues and is necessarily of low vitality. We have already 
alluded to the difference in the teeth of different animals, and 
their relation to customary food and digestive functions. In 
fact, it is clear that the teeth and all the parts of the digestive 


system are correlated to one another. The compound stomach 
of the ruminants, with its slow digestion of a bulky mass of 
food, which must he softened and thoroughly masticated be- 
fore the digestive juices can attack it successfully, harmonizes 
with the powerful jaws, strong muscles of mastication, and 
grinding teeth; and all these in marked contrast with the 
teeth of a carnivorous animal with its simple but highly effect- 
ive stomach. Compare figures in earlier pages. 

Mastication in man is of 'that intermediate character befit- 
ting an omnivorous animal. The jaws have a lateral and 
forward-and-backward movement, as well as a vertical one, 
though the latter is predominant. The upper jaw is like a 
fixed millstone, against which the lower jaw works as a nether 
millstone. The elevation of the jaw is effected by the mas- 
seter, temporal, and internal pterygoid muscles ; depressed by 
the mylohyoid and geniohyoid, though principally by the di- 
gastric. The jaw is advanced by the external pterygoids; 
unilateral contractio'n of these muscles also produces lateral 
movement of the inferior maxilla, which is retracted by the 
more horizontal fibers of the temporal. 

The cheeks' and tongue likewise take part in preparing the 
food for the work of the stomach, nor must the lips be over- 
looked even in man. The importance of these parts is well 
illustrated by the imperfect mastication, etc., when there is 
paralysis of the muscles of which they are formed. Even when 
there is loss of sensation only, the work of the mouth is done 
in a clumsy way, showing the importance of common sensation, 
as well as the muscular sense. 

Nervous Supply. — The muscles of the tongue are governed by 
the hypoglossal nerve ; the other muscles of mastication chiefly 
by the fifth. The afferent nerves are branches of the fifth and 
glosso-pharyngeal. It is, of course, important that the food 
should be rolled about and thoroughly mixed with saliva (in- 

Deglutition. — The transportation of the food from the mouth 
to the stomach involves a series of co-ordinated muscular acts 
of a complicated character, by which difficulties are overcome 
with marvelous success. 

It will be remembered that the respiratory and digestive 
tracts are both developed from a common simple tube — a fact 
which makes the close anatomical relation between these two 
physiologically distinct systems intelligible ; but it also involves 
difficulties and dangers. It is well known that a small quantity 



of food or drink entering the windpipe produces a perfect 
storm of excitement in the respiratory system. The food, there- 

Fio. 281.— Cavities of mouth and pharynx, etc. (after Sappey). Section, in median Une, of 
face and superior portion of neclc, designed to show the mouth in its relations to the nasal 
fossae, pharynx, and larynx : 1, sphenoidal sinuses ; 2, internal orifice of Eustachian tube : 
3, palatine arch ; 4, velum pendulum palati : 5, anterior pillar of soft palate ; 6, posterior 
pillar of soft palate ; 7, tonsil ; 8, lingual portion of cavity of pharynx ; 9, epiglottis ; 10, 
section of hyoid bone ; II, laryngeal portion of cavity of pharynx ; 12, cavity of larynx. 

fore, when it readies the oesophagus, must be kept, on the one 
hand, from entering the nasal, and, on the otlier, tlie laryngeal 
openings. This is accomplished as follows : When the food has 
been gathered into a bolus on the back of the tongue, the tip of 
this organ is pressed against the hard palate, 'by which the 
mass is prevented from passing forward, and, at the same time, 
forced back into the pharynx, the soft palate being raised and 
the edges of the pillars of the fauces made to approach the 
uvula, wliich fills up the gap remaining, so that the posterior 
nares are closed and an inclined plane provided, over which 
the morsel glides. The after-result is said to depend on the 
size of the bolus. When considerable, the constrictors of the 


pharynx seize it and press it on into the gullet ; when the mor- 
sel is small' or liquid is swallowed, it is rapidly propelled on- 
ward by the tongue, the oesophagus and pharynx being largely 
passive at the time, though contracting slowly afterward ; at 
the same time the larynx as a whole is raised, the epiglottis 
pressed down, chiefly by the meeting of the tongue and itself, 
while its cushion lies over the rima glottidis, which is closed or 
all but closed by the action of the sphincter muscles of the 
larynx, so that the food passes over and by this avenue of life, 
not only closed but covered by the glottic lid. The latter is 
not so essential as might be supposed, for persons in whom it 
was absent have been known to swallow fairly well. The 
ascent of the larynx any one may feel for himself; and the be- 
havior of the pharynx and larynx, especially the latter, may 
be viewed by the laryngoscope. The grip of the pharyngeal 
muscles and the oesophagus may be made clear by attaching a 
piece of food (meat) to a string and allowing it to be partially 

The upward movement of food under the action of the con- 
strictors of the pharynx is anticipated by the closure of the 
passage by the palato-glossi of the anterior pillars of the fauces. 

The circular muscular fibers of the gullet are probably the 
most important in squeezing on the food by a peristaltic move- 
ment, passing progressively over the whole tube, though the 
longitudinal also take part in swallowing, perhaps, by steady- 
ing the organ. 

Swallowing will take place in an animal so long as the 
medulla oblongata remains intact ; and the center seems to lie 
higher than that for respiration, as the latter act is possible 
when,' from slicing away the medulla, the former is not. An- 
encephalous monsters lacking the cerebrum can swallow, suck, 
and breathe. 

Food placed in the pharynx of animals when unconscious 
is swallowed, proving that volition is not essential to the act ; 
but our own consciousness declares that the first stage, or the 
removal of the food from the mouth to the pharynx, is volun- 

When we seem to swallow voluntarily there is in reality a 
stimulus applied to the fauces, in the absence of food and drink, 
either by the back of the tongue or by a little saliva. 

It thus appears that deglutition is an act in the main reflex, 
though initiated by volition. The afferent nerves concerned 
are usually the glosso-pharyngeal, some branches of the fifth. 



and of the vagus. The efferent nerves are those of the numer- 
ous muscles concerned. 

When food has reached the gullet, it is, of course, no longer 
under the control of the will. 

Section of the vagus or stimulation of this nerve modifies 
the action of the oesophagus, though it is known that contrac- 
tions may be excited in the excised organ ; but no doubt nor- 
mally the movements of the gullet arise in response to natural 
nerve stimulation. 

Comparative. — That swallowing is independent of gravity is 
evident from the fact that long-necked animals (horse, giraffe) 
can and do usually swallow with the head and neck down, so 
that the fluid is rolled up an inclined plane. The peristaltic 
nature of the contractions of the gullet can also be well seen 
in such animals. In the frog the gullet, as well as the mouth, 
is lined with ciliated epithelium, so that in a recently killed 
animal one may watch a slice of moistened cork disappear from 
the mouth, to be foiind shortly afterward in the stomach. The 
rate of the descent is surprising — in fact, the movement is 
plainly visible to the unaided eye. 

The Movements of the Stomach. — The stomach of mammals, 
including man, is provided with three layers of muscular fibers : 
1. External longitudinal, a continuation of those of the oesopha- 
gus. 2. Middle circular. 3. Internal oblique. The latter are 

Fig. 282. — Human stomach (after Sappey). 1, oesophagus : 2. circular fibers at oesophageal 
opening ; .3, 3, circular fibers at lesser curvature ; 4, 4, circular fibers at the pylorus ; 5, 5, 
6, 7, 8, oblique fibers ; 9, 10, fibers of this layer covering the greater pouch ; 11. portion of 
stomach from which these fibers have been removed to show the subjacent circular fibers. 


the least perfect, viewed as an investing coat. The pyloric end 
of the stomach is best supplied with muscles ; where also there 
is a thick muscular ring or sphincter, as compared with which 
the cardiac sphincter is weak and ill-developed. 

The movements of the stomach begin shortly after a meal 
has been taken, and, as shown by observations on St. Martin, 
continue for hours, not constantly, but periodically. The effect 
of the conjoint action of the different sets of muscular fibers is 
to move the food from the cardiac toward the pyloric end of 
the stomach, along the greater curvature and back by the lesser 
curvature, while there is also, probably, a series of in-and-out 
currents to and from the center of the food-mass. The quantity 
of food is constantly being lessened by the removal of digested 
portions, either by the blood-vessels of the organ or by its 
passing through the pyloric sphincter. The empty stomach is 
quiescent and contracted, its mucous membrane being thrown 
into folds. 

The movements of the stomach may be regarded as reflex, 
the presence of food being an exciting cause, though probably 
not the only one ; and so largely automatic is the central mech- 
anism concerned, that but a feeble stimulus suffices to arouse 
them, especially at the accustomed time. 

Of the paths of the impulses, either afferent or efferent, 
little is known. Certain effects follow section or stimulation of 
the vagi or splanchnics, but these can not be predicted with 
certainty, or the exact relation of events indicated. 

It is said that the movements of the stomach cease, even 
when it is full, during sleep, from which it is argued that gas- 
tric movements do normally depend on the influence of the 
nervous system. However, the subject is too obscure at pres- 
ent for further discussion. 

Comparative. — Recent investigations on the stomach of the 
pig indicate that in this animal the contents of the two ends of 
the stomach may long remain but little mingled ; and such is 
certainly the case in. this organ among ruminants. 

Pathological. — Distention of the stomach, either from excess 
of food or gas arising from fermentative changes, or by secre- 
tion from the blood, may cause, by upward pressure on the 
diaphragm, etc., uneasiness from hampered respiration, and ir- 
regularity of the heart, possibly, also, in part traceable to the 
physical interference with its movements. After great and 
prolonged distention there may be weakened digestion for a 
considerable interval. It seems not improbable that this is to 


be explained, not alone by the impaired elasticity (vitality) of 
the muscular tissue, but also by defective secreting power. It 
is not necessary to impress the lesson such facts convey. 

The Intestinal Movements. — The circular fibers play a much 
more important part than the longitudinal, being, in fact, much 
more developed. It is also to be remembered that nerves in 
the form of plexuses (of Auerbach and Meissner) abound in its 

Normally the movement, slowly progressive, with occasional 
baitings, is from above downward, stopping at the ileo-csecal 
valve ; the movements of the large gut being apparently mostly 

Movements may be excited by external or internal stimula- 
tion, and may be regarded as reflex ; in which, however, the 
tendency for the central cells to discharge themselves is so 
great (automatic) that only a feeble stimulus is required, the 
normal one being the presence of food. 

It is noticeable in a recently killed animal, or in one in the 
last stages of asphyxia, that the intestines contract vigorously. 
Whether this is due to the action of blood overcharged with 
carbonic anhydride and deficient in oxygen on the centers pre- 
siding over the movements, on the nerves in the intestinal 
walls, or on the muscle-cells directly, is not wholly clear, but it 
is probable that all of these may enter into the result. The 
vagus nerve, when stimulated, gives rise to movements of the 
intestines, while the splanchnic seems to have the reverse effect ; 
but the cerebrum itself has an influence over the movements of 
the gut, as is plain from the diarrhoea traceable to unusual 
fear or anxiety. There is little to add in regard to the move- 
ments of the large intestine. They are, no doubt, of consider- 
able importance in animals in which it is extensive. Normally 
they begin at the ileo-caecal valve. 

Defecation. — The removal of the waste matter from the ali^ 
mentary tract is a complicated process, in which both smooth 
and striped muscle, the spinal cord, and the brain take part. 

Defecation may take place during the unconsciousness of 
sleep or of disease, and so be wholly independent of the will ; 
but, as we well know, this is not usually the case. Against ac- 
cidental discharge of faeces there is a provision in the sphinc- 
ter ani, the tone of which is lost when the lower part of the 
spinal cord is destroyed. We are conscious of being able, by an 
effort of will, to prevent the relaxation of the sphincter or to 
increase its holding power, though the latter is probably almost 


wholly due to the action of extrinsic muscles ; at all events any 
one may convince himself that the latter may he made to take 
a great part in preventing faecal discharge, though whether the 
tone of the sphincter can be increased or not by volition it is 
difficult to say. 

What happens during an ordinary act of defecation is about 
as follows : After a long inspiration the glottis is closed ; the 
diaphragm, which has descended, remains low, affording, with 
the obstructed laryngeal outlet, a firm basis of support for the 
action of the abdominal muscles, which, bearing on the intes- 
tine, forces on their contents, which, before the act has been 
called for, have been lodged mostly in the large intestine ; at 
the same time the sphincter ani is relaxed and peristaltic move- 
ments accompany and in some instances precede the action of 
the abdominal muscles. The latter jnay contract vigorously on 
a full gut without success in the absence of the intestinal peri- 
stalsis, as too many cases of obstinate constipation bear witness. 

Like deglutition, and unlike vomiting, there is usually both 
a voluntary and involuntary part to the act. 

Though the will, through the cerebrum, can inhibit defeca- 
tion, it is likely that it does so through the influence of the 
cerebrum on some center in the cord ; for in a dog, the lumbar 
cord of which has been divided from the dorsal, the act is, like- 
micturition, erection of the penis, and others which are under the 
control of the will, still possible, though, of course, performed 
entirely unconsciously. 

Vomiting. — If we consult our own consciousness and observe 
to the best of our ability, supplementing information thus 
gained by observations on others and on the lower animals, it 
will become apparent that vomiting implies a series of co-ordi- 
nated movements, into which volition does not enter either 
necessarily or habitually. There is usually a preceding nausea, 
with a temporary flow of saliva to excess. The act is initiated 
by a deep inspiration, followed by closure of the glottis. 
Whether the glottis is closed during or prior to the entrance 
of air, is a matter of disagreement. At all events, the dia- 
phragm descends and remains fixed, the lower ribs being re- 
tracted. The abdominal muscles then acting against this sup- 
port, force out the contents of the stomach, in which they are 
assisted by the essential relaxation of the cardiac spTiincter, the 
shortening of the oesophagus by its longitudinal fibers, and the 
extension and straightening of the neck, together with the open- 
ing of the mouth. 


As tlie expulsive effort takes place, it is accompanied by an 
expiratory act wMch tends to keep the egesta out of the larynx 
and carry them onward, though it may also contribute to over- 
come the resistance of the elevated soft palate, which serves to 
protect the nasal passages. The stomach and CBSophagus are 
not wholly passive, though their part is not so important in 
the adult as might be inferred from observing vomiting in 
infants, the peristalsis of these organs apparently sufficing in 
them to empty the stomach. 

Retching may be very violent and yet ineffectual when the 
cardiac sphincter is not fully relaxed. The pyloric outlet is 
usually closed, though in severe and long-continued vomiting 
bile is often ejected, which must have reached the stomach 
through the pylorus. 

Comparative. — The ease with which some animals vomit in 
comparison with others is extraordinary, as in carnivora like 
our dogs and cats ; a matter of importance to an animal ac- 
customed in the wild state to eat entire carcasses of animals — 
hair, bones, etc., included. 

The readiness with which an animal vomits depends in great 
part on the conformation and relations of the parts of its digest- 
ive tract. 

The horse vomits with difficulty — its stomach and its car- 
diac opening being small and peculiar in shape (Figs. 261 and 
280), while its oesophagus is long. The stomach of the human 
being during infantile life is less pouched than in the adult, 
which in part explains the ease with which infants vomit. 

But the matter is complex; much depends on the proper 
co-ordinations being made, and, this being well or ill accom- 
plished, accounts for the variations in the ease with which dif- 
ferent persons vomit. 

Pathological. — Vomiting may arise from the presence of renal 
or biliary calculi (reflex action) ; from disease of the cerebrum 
or the medulla ; from obstruction in the pyloric region or in 
the intestines ; from emotions ; from revived unpleasant men- 
tal associations ; from nauseous tastes, etc. It may be- ques- 
tionable whether some of these are properly termed " patho- 

Pyrosis is due to the anti-peristaltic action of the stomach 
and oesophagus alone, so that it is a sort of partial vomiting 
and allied to the regurgitation of special secretions, as from the 
crops of pigeons, or of food from the stomachs of ruminants. 
We have known cases in which anti-peristalsis was confined to 


the pharynx alone. Some persons seem to have acquired the 
power of regurgitating food and masticating it afresh. 

The excessive vomiting following obstruction of the bowels 
is comparable to the unusual action of the heart, ureter, blad- 
der, etc., when there is hindrance to the outflow. As we have 
already explained for the heart, we regard this as the resump- 
tion of a power of independent action seen ' in ancestral forms 
and marked when the nervous system is no longer exercising 
its usual control and direction. Not that this or similar be- 
havior may not result from excessive stimulation, leading to 
unusual central nervous discharge, but it certainly does happen 
independently of the nervous system, and may be witnessed in 
the hearts of cold-blooded animals when all their nerves are 

Similarly, the habit of regurgitating the food is intelligible 
in the light of evolution. The fact that mammals are descended 
from lower forms in which unstriped muscle-cells go to form 
organs that have a rhythmically contractile function, renders 
it clear why this function may become, as in ruminants, spe- 
cialized in certain parts of the digestive tract ; why carnivora 
should vomit readily, and why human subjects should learn to 
regurgitate food. There is, so to speak, a latent inherited ca- 
pacity which may be developed into actual function. Apart 
from this it is difiicult to understand such cases at all. 

The vomiting center is usually located in the medulla, and 
is represented as working in concert with the respiratory center. 
But when we consider that there is usually an increased flow 
of saliva and other phenomena involving additional central 
nervous influence, we see reason to believe in co-ordinated 
action implying the use of parts of the central nervous system 
not so closely connected anatomically as the respiratory and 
vomiting centers are assumed to be. 

Indeed, as we before indicated, it does not seem probable 
that the doctrine of centers in its present form, especially with 
such precise limitations, both anatomically and physiologically, 
will continue to be maintained. We seem to have been over- 
looking the connection of parts while occupied with defining 
their limits. It is not, however, yet possible to substitute 
other explanations that shall be wholly satisfactory ; and we 
make these remarks to keep the student expectant of progress, 
for, as a distinguished exponent of science has said, '''When 
Science adopts a [rigid] creed, she commits suicide." 

We do not know the part taken, if any, by the splanchnic 


or other nerves of the sympathetic system ; hut, from the fact 
that discharge of the gastric contents is impossible when the 
vagi are cut, it is likely that the efferent impulses, determining 
the relaxation of the cardiac sphincter, descend by these nerves, 
while the chorda tympani is concerned, of course, in the secre- 
tion of saliva. But it will be clear, from the facts of the case, 
that many nerves, both afferent and efferent, are concerned; 
and it is more than likely that our explanations of the entire 
process are quite inadequate to unravel its real complexity. 

Therapeutics. — The evidence from the use of drugs seems to 
emphasize the last statement. At all events, emetics act in a 
variety of ways, and differently in different animals. 

The Removal of Digested Products from the Aliment- 
ary Canal. 

The glands of the stomach are simply secretive, and all ab- 
sorption from this organ is either by blood-vessels directly or 
by lymphatics ; at least, such is the ordinary view of the sub- 
ject — whether it is not too narrow a one remains to be seen. 

It is important to remember that the intestinal mucous 
membrane is supplied not only with secreting glands but lym- 
phatic tissue, in the form of the solitary and agminated glands 
(Peyer's patches) and thic^jily studded with villi, giving the 
small gut that velvety appearance appreciable even by the 
naked eye. 

It will not be forgotten that the capillaries of the digestive 
organs terminate in the veins of the portal system, and that the 
blood from these parts is conducted through the liver before it 
reaches the general circulation. 

The lymphatics of these organs form a part of the general 
lymphatic system of the body ; but the peculiar way in which 
absorption is effected by villi, and the fact that the lymphatics 
of the intestine, etc., at one time (fasting) contain ordinary 
lymph and at another (after meals) the products of digestion, 
imparts to them a physiological character of their own. 

Absorption will be the better understood if we treat now of 
lymph and chyle and the lymph vascular system, which were 
purposely postponed till the present; though its connection 
with the vascular system is as close and important as with 
the digestive organs. 

The lymphatic system, as a whole, more closely resembles 
the venous than the arterial vessels. We may speak of lym- 



phatic capillaries, which are, in essential points 
of structure, like the arterial capillaries ; while 
the larger vessels may be compared to veins, 
though thinner, being provided with valves and 
having very numerous anastomoses. These 
lymphatic capillaries begin in spaces between 
the tissue-cells, from which they take up the 
effete lymph. It is interesting to note that 
there are also perivascular lymphatics, the ex- 
istence of which again shows how close is the 
relation between the blood vascular and lym- 
phatic systems, and as we would suppose, and 
as is actually found to be the case, between the 
contents of each. 

Lymph and Chyle. — If one compares the mes- 
entery in a kitten, when fasting, with the same 
part in an animal that was killed some hours 
after a full meal of milk, it may be seen that 
the formerly clear lines indicating the course of 
Fig. 283.— Valves ot file lymphatics and ending in glands have in 
lymphatics (Sappey). ^-^^ j^^^-gj. pj^gg become whitish (hence their 

name, lacteals), owing to the absorption of the emulsified fat of 
the milk. 

Fig. 284.— Origin of lymphatics (after Landois). I. From central tendon of diaphragm of 
rabbit (semi-diagrammatic) ; s, lymph-canals communicating by X with lymphatic vessel 
L ; A^ origin of lymphatic by union of lymph-canals ; E, E^ endothehum. U. Perivas- 
cular canal. 



Microscopic examination shows the chyle to contain (when 
coagulated) fibrin, many leucocytes, a few developing red cor- 
puscles, an abundance of fat in the form both of very minute 
oil-globules and particles smaller still. 

Fig. 285.— Epithelium from duodenum of rab- 
bit, two hours after having been fed with 
melted butter (Funke), 

Fig. 286.— VilU filled with fat, from small 
intestine of an executed criminal, one 
hour after death (Funke). 

There are also present fatty acids, soaps small in quantity 
as compared with the neutral fats, also a little cholesterin and 
lecithin. But chyle varies very 
widely even in the same animal 
at different times. To the above 
must be added proteids (fibrin, 
serum-albumin, and globulin) ; 
extractives (sugar, urea, leu- 
cin) ; and salts in which sodium 
chloride is abundant. 

The composition of lymph is 
so similar to that of chyle, and 
both to blood, that lymph 
might, with a fair degree of ac- 
curacy, be regarded as blood 
without its red corpuscles, and 
chyle as lymph with much neu- 
tral fat in a very fine state of 

The Movements of the Lymph — comparative. — In some fishes, 
some birds, and amphibians, there are lymph hearts. 

In the frog there are two axillary and two sacral lymph 
hearts. The latter are, especially, easily seen, and there is no 
doubt that they are under the control of the nervous system. 

Fig. 287.— Chyle taken from the lacteals 
and thoracic duct of a criminal exe- 
cuted during digestion {Funke). Shows 
leucocytes and excessively fine granules 
of fatty emulsion. 



In the mammals no such special helps for the propulsion of 
lymph exist. 

There is little doubt that the blood - pressure is always 
higher than the lymph-pressure, and when the blood-vessels 

Fig. 288. — Thoracic duct (Mascag:m) 1 thoncic duct 2 great lymphatic duct; 3, recep- 
taculum chyli ; 4, curve of thoracic duet ]u&t before it empties into the venous system. 

are dilated the fluid within the perivascular lymph-channels is 
likely compressed ; muscular exercise must act on the lymph- 
channels as on veins, both being provided with valves, though 
themselves readily compressible ; the inspiratory efforts, espe- 
cially when forcible, assist in two ways : by the compressing 
effect of the respiratory muscles, and by the aspirating effect 
of the negative pressure within the thorax, producing a similar 
aspirating effect within the great veins, into which the large 
lymphatic trunks empty. The latter are provided at this point 
with valves, so that there is no back-flow ; and, with the posi- 
tive pressure within the large lymphatic trunks (thoracic duct, 
etc.), the physical conditions are favorable to the outflow of 
lymph or chyle. 



Out knowledge of the nature of the passage of the chyle 
from the intestines into the blood is now clearer than it was till 
recently, though still incomplete. 

The exact structure of a villus is to be carefully considered. 
If we assume that the muscular cells in its structure have a 
rhythmically contractile function, the blind terminal portion 
of the lacteal inclosed within the villus must, after being 
emptied, act as a suction-pump to some extent ; at all events, 
the conditions as to pressure would be favorable to inflow of 
any material, especially fluid without the lacteal. The great 
difficulty hitherto was to understand how the fat found its 

Fio. 289.— Lymphatic vessels and glands (Sappey). 1, upper extremitj- of thoracic duct, pass- 
inp; behind the internal ju^lar vein ; 2. opening of thoracic duct into internal jugular and 
left subclavian vein. Lymphatic glands are seen in course of vessels. 

way through the villus into the blood, for, that most of it 
passes in this direction there is little doubt. 

It is now known that leucocytes (amoeboids, phagocytes) 
migrate from within the villus outward, and may even reach 
its surface ; that they take up (eat) fat-particles from the 



Fro. 290.— Stomach, intestine, and mesentery, with mesenteric blood-vessels and lacteals 
(slightly reduced from a figure in the original work of Asellius, published in 1628) (after 
Flint). A, A, A, A, A, mesenteric arteries and veins ; B, B, B, B, B, B, B, B, B, B, lacteals ; 
C, C, C, C, mesentery ; A D, stomach ; E, pyloric portion of stomach ; F, duodenum ; 
G, G, G, jejunum ; H, K, H, H, H, ileum ; /, artery and vein on fxmdus of stomach ; K^ 
portion of omentum. 

epithelium of the villus, and, independently themselves, carry 
them inward, reach the central lacteal and break up, thus releas- 
ing the fat. How the fat gets into the covering epithelium is 



not yefc so fully known — possibly by a, simi- 
lar inceptive process ; nor is it ascertained 
"what constructive or other chemical pro- 
cesses they may perform ; though it is not 
at all likely that the work of the amoeboid 
cells is confined to the transport of fat 
alone, but that other matters ar« also thus 
removed inward to the lacteal. 

Experimental.— If two frogs under the 
influence of urari, to remove the effect of 
muscular movements, be placed under ob- 
servation, the one having its brain and 
spinal cord destroyed, the other intact, in 
both the aorta divided across, and normal 
saline solution injected into the posterior 
lymph-sac (beneath the skin of the back), 
it will be found, on suspending the two 
by the lower jaw, that, in the frOg with 
the nerve - centers uninjured, abundance 
of saline fluid is taken up from the dor- 
sal sac and expelled through the aorta, 
but in the other case none, the heart remaining all but empty, 

Fio. 291.— Intestinal villus 
(after Leydig). a, a, a, 
epithelial covering ; &, 

b, capillary network ; 

c, c, longitudinal mus- 
cular fibers : d, lacteal. 

Fia. 392.— A. Villi of man, showing blood-vessels and lacteals; 


B. Villus of sheep (after 



Different interpretations have been put upon this experi- 
ment. Some point to it as clear proof of the influence of the 


Fig. 293. — A. Section of villus of rat killed during fat absori>tion (Schafer). ep. epithelium ; 
str, striated border ; c, lymph-cells ; c', lymph-cells in epithelium ; I, central lacteal con- 
taining disintegrating corpuscles. B. Ilucous membrane of frog's intestine during fat 
absorption (Schafer). ep, epithelium; sti-, striated border; C, lymph-corpuscles; I, lacteal. 

nervous system directly ; to others it seems that the failure of 
absorption is owing to the greatly dilated condition of the 
blood-vessels, consequent upon the loss of arterial tone, the 
blood remaining in the veins, and the circulation being, in fact, 
practically arrested. It certainly can not be claimed that the 
first conclusion necessarily follows from the experiment; the 
second may be a partial explanation of the failure of absorp- 
tion; but, when a multitude of other facts are taken into 
account, there seems little reason to doubt that so important a 
process as absorption can not fail to be regulated by the nerv- 
ous centers. The danger of founding any important conclu- 
sion on a single experiment is very great. 

Again, if the leg of a frog, exclusive of the nerves, be liga- 
tured, the limb will be found to swell rapidly if placed in water, 
which is not true of a dead limb. This is adduced as evidence 
for the independence of the absorptive process and the circula- 
tion; and, since section of the sciatic nerve is said to arrest 
absorption, such an experiment, taken together with the two 


previous ones, points in the direction of the control of this 
process by the nervous system. But if the views we hold of 
the absolute dependence, especially in the higher animals, of 
all vital processes on the nervous system are correct, it fol- 
lows, as a matter of course, that absorption in living tissues, 
which we do not regard as wholly explicable by any physical 
process, but as bound up with all the functions of cell-life, 
must be dependent on that connection we are endeavoring to 
emphasize between one tissue and another, and especially the 
dominating tissue, the nervous system. 

There are two points that are very far from being deter- 
mined : the one the fate of the products of digestion ; the other 
the exact limit to which digestion is carried. How much — 
e. g., of proteid matter — does actually undergo conversion into 
peptone ; how much is converted into leucin and tyrosin ; or, 
again, what proportion of the albuminous matters are dealt with 
as such by the intestine without conversion into peptone at all, 
either as soluble proteid or in the form of solid particles ? 

1. It is generally believed that soluble sugars are absorbed, 
usually after conversion into maltose or glucose, by the capil- 
laries of the stomach and intestine. 

2. There is some positive evidence of the presence of fats, 
soaps, and sugars in unusual amount after a meal in the portal 
vein, which implies removal from the intestinal contents by 
the capillaries, though, so far as experiment goes, the fat is 
chiefly in the form of soaps. 

Certain experiments have been made by ligating the pyloric 
end of the stomach, by introducing^ a cannula into the thoracic 
duct, so as to continually remove its contents, etc. But we are 
surprised that serious conclusions should have been drawn under 
such circumstances, seeing that the natural conditions are so 
altered. What we wish to get at in physiology is the normal 
function of parts, and not the possible results after our inter- 
ference. Under such circumstances the phenomena may have 
a suggestive but certainly can not have a conclusive value. 

It is a very striking fact that little peptone (none, according 
to some observers) can be detected even in the portal blood. 
True it is, the circulation is rapid and constant, and a small 
quantity might escape detection, yet a considerable amount be 
removed from the intestine in the space of a few hours by the 
capillaries alone. Peptone is not found in the contents of the 
thoracic duct. 

Recent investigations have thrown a new light on peptone. 


It is now known that there are several kinds of peptones, a 
disclosure for which we were not unprepared, considering our 
imperfect knowledge of proteids in general ; but there have 
been other developments which, on the supposition that the 
peptone of the alimentary canal is freely absorbed as such, are 
startling enough. It has been shown that these peptones, at 
least as prepared by artificial digestion, have three effects when 
injected in quantity into the blood of an animal : They produce 
narcosis ; they retard or prevent coagulation of the blood ; they 
lower blood-pressure. The first effect may be dependent in 
whole or in part on the third. 

But, inasmuch as the venom of poisonous reptiles, according 
to recent investigations, is essentially proteid in nature, it is 
plain that we must exercise great caution in drawing conclu- 
sions in regard to the physiological effects of proteid bodies, so 
long as our knowledge of their exact chemical composition is 
so imperfect. That the chemist can make out no great differ- 
ence between peptones prepared in the laboratory and the di- 
gestive tract, or even between these and snake- venom, though 
they have such different effects when injected into the blood, 
is clear proof of how much we have yet to learn of these 

But we introduce these considerations here rather to show 
that it is by no means likely that any great quantity of pep- 
tones passes into the blood as such at any one time. It has 
been recently suggested that peptone is converted into globulin 
in the liver. But what proof is there of this ? And already 
we have credited the liver with a large share of work. 

For a considerable period it has been customary to use the 
terms endosmosis and diffusion in connection with the func- 
tions of the alimentary canal, and especially the intestinal tract, 
as if this thin- walled but complicated organ, or rather collec- 
tion of organs, were little more, so far as absorption is con- 
cerned, than a moist membrane, leaving the process of the re- 
moval of digested food products to be explained almost wholly 
on physical principles. 

From such views we dissent. We believe they are opposed 
to what we know of living tissue everywhere, and are not sup- 
ported by the special facts of digestion. When certain foreign 
bodies (as purgatives) are introduced into the blood or the ali- 
mentary canal, that diffusion takes place, according to physical 
laws, may indicate the manner in which the intestine can act ; 
but even admitting that under such circumstances physical 


principles actually do explain the whole, which we do not grant, 
it would by no means follow that such was the natural behav- 
ior of this organ in the discharge of its ordinary functions. 

When we consider that the blood tends to maintain an equi- 
librium, it must be eAddent that the removal of substances from 
the alimentary canal, unless there is to be excessive activity of 
the excretory organs and waste of energy both by them and 
the digestive tract, must in some degree depend on the demand 
for the products of digestion by the tissues. That there is to 
some extent a corrective action of the excretory organs always 
going on is no doubt true, and that it may in cases of emergency 
be great is also true ; but that this is minimized in ways too 
complex for us to follow in every detail is equally true. Diges- 
tion waits on appetite, and the latter is an expression of the 
needs of the tissues. We believe it is literally true that in a 
healthy organism the rate and character of digestion and of 
the removal of prepared products are largely dependent on the 
condition of the tissues of the body. 

Why is digestion more perfect in overfed individuals after 
a short fast ? The whole matter is very complex, but we think 
it is infinitely better to admit ignorance than attempt to ex- 
plain by principles that do violence to our fundamental con- 
ceptions of life processes. To introduce " ferments " to explain 
so many obscure points in physiology, as the conversion of 
peptone in the blood, for example, is taking refuge in a way 
that does no credit to science. 

Without denying that endosmosis, etc., may play a part in 
the vital processes we are considering, we believe a truer view 
of the whole matter will be ultimately reached. In the mean 
time we think it best to express our belief that we are ignorant 
of the real nature of absorption in great part ; but we think 
that, if the alimentary tract were regarded as doing for the 
digested food (cTiyle, etc.) some such work as certain other 
glands do for the blood, we would be on the way to a truer con- 
ception of the real nature of the processes. 

It would then be possible to understand that proteids either 
in the form of soluble or insoluble substances, including pep- 
tone, might be taken in hand and converted by a true vital 
process into the constituents of the blood. 

If we were to regard the kidney as manufacturing useful 
instead of harmful products, the resemblance in behavior would 
in many points be parallel. We have seen that mechanical 
explanations of the functions of the kidney have failed, and 


that it must be regarded even iu those parts that eliminate 
most water as a genuine secreting mechanism. 

We wish to present a somewhat truer conception of the 
lymph that is separated from the capillaries and bathes the 

We would regard its separation as a true secretion, and not 
a mere diffusion dependent wholly on blood-pressure. The 
mere ligature of a vein does not suffice to cause an excess of 
diffusion, but the vaso-motor nerves have been shown to be 
concerned. The effusions that result from pathological pro- 
cesses do not correspond with the lymph — that is,- the nutrient 
material — provided by the capillaries for the tissues. These 
vessels are more than mere carriers ; they are secretors — in a 
sense they are glands. We have seen that in the foetus they 
function both as respiratory and nutrient organs in the allan- 
tois and yelk-sac, and, in our opinion, they never wholly lose 
this function. 

The kind of lymph that bathes a tissue, we believe, depends 
on its nature and its condition at the time, so that, as we view 
tissue-lymph, it is not a mere effusion with which the tissues, 
for which it is provided, have nothing to do. The differences 
may be beyond our chemistry to determine, but to assume that 
all lymph poured out is alike is too crude a conception to meet 
the facts of the case. Glands, too, it will be remembered, derive 
their materials, like all other tissues, not directly from the 
blood, but from the lymph. We believe that the cells of the 
capillaries, like all others, are influenced by the nervous system, 
notwithstanding that nerves have not been traced terminating 
in them. 

It is to be borne in mind that the lymph, like the blood, 
receives tissue waste-products— in fact, it is very important to 
realize that the lymph is, in the first instance, a sort of better 
blood — an improved, selected material, so far as any tissue is 
concerned, which becomes gradually deteriorated (see Fig. 339). 

We have not the space to give all the reasons on which the 
opinions expressed above are founded ; but, if the student has 
become imbued with the principles that pervade this work thus 
far, he will be prepared for the attitude we have taken, and 
sympathize with our departures from the mechanical (physical) 

We think it would be a great gain for physiology if the use 
of the term " absorption," as applied to the alimentary tract, 
were given up altogether, as it is sure to lead to the substitu- 


tion of the gross conceptions of physical processes instead of 
the subtle though at present rather indefinite ideas of vital 
processes. We prefer ignorance to narrow, artificial, and erro- 
neous views. 

PathologicaL — Under certain circumstances, of which one is 
obstruction to the venous circulation or the lymphatics, fluid 
may be poured out or effused into the neighboring tissues or the 
serous cavities. This is of very variable composition, but always 
contains enough salts and proteids to remind one of the blood. 

Such fluids are often spoken of as "lymph," though the 
resemblance to normal tissue-lymph is but of the crudest kind ; 
and the condition of the vessels when it is secreted, if such a 
term is here appropriate, is not to be compared to the natural 
separation of the normal lymph — in fact, were this not so, it 
would be like the latter, which it is not. When such effusions 
take place they are in themselves evidence of altered (and not 
merely increased) function. . 

The Fseces. — The faeces may be regarded in at least a three- 
fold aspect. They contain undigested and indigestible rem- 
nants, the ferments and certain decomposition products of the 
digestive fluids, and true excretory matters. 

In carnivorous and omnivorous animals, including man, 
the undigested materials are those that have escaped the action 
of the secretions — such as starch and fats — together with those 
substances that the digestive juices are powerless to attack, 
as horny matter, hairs, elastic tissue, etc. 

In vegetable feeders a larger proportion of chlorophyl, cel- 
lulose, and starch will, of course, be found. 

These, naturally, are variable with the individual, the spe- 
cies, and the vigor of the digestive organs at the time. 

Besides the above, certain products are to be detected in the 
faeces plainly traceable to the digestive fluids, and showing 
that they have undergone chemical decomposition in the ali- 
mentary tract, such as cholalic acid, altered coloring-matters 
like urobilin, derivable probably from bilirubin ; also choles- 
terine, fatty acids, insoluble soaps (calcium, magnesium), to- 
gether with ferments, having the properties of pepsin and 
amylopsin. Mucus is also abundant in the faeces. 

We know little of the excretory products proper, as they 
probably normally exist in small quantity, and it is not impos- 
sible that some of the products of the decomposition of the 
digestive juices may be reabsorbed and worked over or excreted 
by the kidneys, etc. 



• There is, however, a recognized non-nitrogenous crystalline 
body known as excretin, which contains sulphur, salts, and 
pigments, and that may rank perhaps as a true excretion of 
the intestine. 

It is well known that bacteria abound in the alimentary 
tract, though their number is dependent on a variety of circum- 
stances, including the kind of food and the condition in which 
it is eaten. These minute organisms feed, of course, and to get 
their food produce chemical decompositions. Skatol and indol 
are possibly thus produced, and give the fascal odor to the con- 
tents of the intestine. But as yet our ignorance of these mat- 
ters is greater than our knowledge — a remark which applies to 
the excretory functions of the alimentary tract generally. 

Pathological. — The facts revealed by clinical and pathological 
study leave no doubt in the mind that the intestine at all events 
may, when other glands, like the kidney, are at fault, undertake 
an unusual share of excretory work, probably even to the length 
of discharging urea. 

Obscure as the subject is, and long as it may be before we 
know exactly what and how matter is thus excreted, we think 
that it will greatly advance us toward a true conception of the 
vital processes of the mammalian body if we regard the ali- 
mentary tract as a collection of organs with both a secreting 
and excreting function ; that what we have been terming ab- 
sorption is in the main, at least, essentially secretion or an allied 
process; and that the parts of this long train of organs are 
mutually dependent and work in concert, so that, when one is 
lacking in vigor or resting to a greater or less degree, the others 
make up for its diminished activity ; and that the whole must 
work in harmony with the various excretory organs, as an 
excretor itself, and in unison with the general state of the econ- 
omy. We are convinced that even as an excretory mechanism 
one part may act (vicariously) for another. 

Of course, in disease the condition of the faeces is an indica- 
tion of the state of the digestive organs ; thus color, consistence, 
the presence of food in lumps, the odor, and many other points 
tell a plain story of work left undone, ill-done, or disordered 
by influences operating from within or from without the tract. 
The intelligent physician acts the part of a qualified inspector, 
surveying the output of a great factory, and drawing conclu- 
sions in regard to the kind of work which the operatives have 



The Changes produced in the Food in the Alimentary 


We liave now considered the method of secretion, the secre- 
tions themselves, and the movements of the various parts of 
the digestive tract, so that a brief statement of the results of 
all this mechanism, as represented by changes in the food, will 
be appropriate. We shall assume for the present that the effects 
of the digestive juices are substantially the same in the body 
as in artificial digestion. 

Among mammals food is, in the mouth, comminuted (except 
in the case of the carnivora, that bolt it almost whole, and the 
ruminants, that simply swallow it to be regurgitated for fresh 
and complete mastication), insalivated, and, in most species, 
chemically changed, but only in so far as starch is concerned. 

Deglutition is the result of the co-ordinated action of many 
muscular mechanisms, and is reflex in nature. The oesophagus 
secretes mucus, which lubricates its walls, and aids mechan- 
ically in the transport of the food from the mouth to the stom- 
ach. In the stomach, by the action of the gastric juice, food 
is further broken up, the proteid covering of fat-cells is digested, 
and the structure of muscle, etc., disappears. Proteid matters 

FiQ. 394.— Matters taken from pyloric portion of stomach of dog during digestion of mixed 
food (after Bernard), a, disintegrated muscular fibers, stuas having disappeared ; b, c, 
muscular fibers in which striae have partly disappeared ; <i,d,d, globules of fat ; e, e, e, 
starch ; g, molecular granules. 


become peptone, and in some animals fat is split up into free 
fatty acid and glycerine ; but the digestion of fat in the stom- 
ach, is very limited at best, and probably does not go on to 
emulsification or saponification. The digestion of starch con- 
tinues in the stomach until the reaction of the food-mass be- 
comes acid. This in the hog may not be far from one to two 
hours, and the amylolytic ferment acts with great rapidity even 
without the body. The food is moved about to a certain ex- 
tent, so as to expose every part freely to the mucous mem- 
brane and its secretions. It is likely that the sugar resulting 
from the digestion of starch, the peptones, and, to some ex- 
tent, the fat formed (if any), is received into the blood from 
the stomach. 

As the partially digested mass (chyme) is passed on into the 
intestine as a result of the action of the alkaline bile, the para- 
peptone, pepsin, and bile-salts are deposited. Certain of the 
constituents of digestion are thus delayed, a portion of the pep- 
sin is probably absorbed, either altered or unaltered, and pep- 
sin is thus got rid of, making the way clear, so to speak, for 
the action of trypsin. At all events, digestion in one part of 
the tract is antagonized by digestion in another, but we must 
also add supplemented. 

The fat, which had been but little altered, is emulsified by 
the joint action of the bile and pancreatic secretion ; a portion 
is saponified, which again helps in emulsification, while an 
additional part, in form but little changed, is probably dealt 
with by the absorbents. 

Proteid digestion is continued, and, besides peptones," ni- 
trogenous crystalline bodies are formed (leucin and tyrosin), 
but under what conditions or to what extent is not known; 
though the quantity is likely very variable, both with the spe- 
cies of animal and the circumstances, such as quantity and 
quality of food ; and it is likely also dependent not a little on 
the rate of absorption. It seems altogether probable that in 
those that use an excess of nitrogenous food more of these 
bodies are formed, and thus give an additional work to the ex- 
creting organs, including the liver. But the absence of albu- 
min from healthy faeces points to the complete digestion of 
proteids in the alimentary canal. Plainly the chief work of 
intestinal digestion is begun and carried on in the upper part 
of the tract, where the ducts of the main glands are to be 

The contents of the intestine swarm with bacteria, though 


tiiese are probably kept under control to some extent by tbe 
bile, the functions of wbich as an antiseptic we have already 

The removal of fats by the villi will be shortly considered. 
The other products of digestion probably find their way into 
the general circulation by the portal blood, passing through 
the liver, which organ modifies some of them in ways to be 
examined later. 

The vaUvuLoe, conniventes greatly increase the surface of the 
intestine, and retard the movements of the partially digested 
mass, both of which are favorable. The peristaltic movements 
of the small gut serve the obvious purpose of moving on the 
digesting mass, thus making w:ay for fresh additions of chyme 
from the stomach, and carrying on the more elaborated con- 
tents to points where they can receive fresh attention, both 
digestive and absorptive. 

Comparative. — In man, the carnivora, and some other groups, 
it is likely that digestion in the large intestine is slight, the 
work being mostly completed — at all events, so far as the action 
of the secretions is concerned — ^before this division of the tract 
is reached, though doubtless absorption goes on there also. 
The muscular strength of this gut is important in the act of 

But the great size of the large intestine in ruminants — in 
the horse, etc. — ^together with the bulky character of the food 
of such animals, points to the existence of possibly extensive 
processes of which we are ignorant. It is generally believed 
that food remains but a short time in the stomach of the horse, 
and that the caecum is a sort of reservoir in which digestive 
■processes are in progress, and also for water. 

Fermentations go on in the intestine, and probably among 
ruminants they are numerous and essential, though our actual 
knowledge of the subject is very limited. 

The gases found in the human stomach are atmospheric air 
(swallowed) and carbon dioxide, derived from the blood. Those 
of the intestine are nitrogen, hydrogen, carbonic anhydride, 
sulphureted hydrogen, and marsh-gas, the quantity varying 
considerably with the diet. 

Faithological. — In subjects of a highly neurotic temperament 
and unstable nervous system it sometimes happens that im- 
mense quantities of gas are belched from an empty stomach or 
distend the intestines. 

It is known that the oxygen swallowed is absorbed into the 


blood, and ■ the carbonic anhydride found in the stomaeh de- 
rived from that fluid. 

It will thus be seen that the alimentary tract has not lost 
its respiratory functions even in man, and that these may in 
certain instances be inordinately developed {reversion). 

Special Considerations. 

It is a matter well recognized by those of much experience 
in breeding and keeping animals with restricted freedom and 
under other conditions differing widely from the natural ones 
— i. e.. those under which the animals exist in a wild state — that 
the nature of the food must vary from that which the untamed 
ancestors of our domestic animals used. Food may often with 
advantage be cooked for the tame and confined animal. The 
digestive and the assimilative powers have varied with other 
changes in the organism brought about by the new surround- 
ings. So much is this the case, that it is necessary to resort to 
common experience and to more exact experiments to ascertain 
the best methods of feeding animals for fattening, for work, 
or for breeding. Inferences drawn from the feeding habits of ' 
wild animals allied to the tame to be valuable must always, 
before being applied to the latter, be subjected to correction 
by the results of experience. 

To a still greater degree does this apply to man himself. 
The greater his advances in civilization, the more he departs 
from primitive habits in other respects, the more must he de- 
part in his feeding. With the progressive development of 
man's cerebrum, the keener struggle for place and power, the 
more his nervous energies are diverted from the lower func- 
tions of digestion and assimilation of food ; hence the greater 
need that food shall be more carefully selected, and more 
thoroughly and scientifically prepared. Not only so, but, with 
our increasing refinement, the progress of digestion to suc- 
cessful issues demands that the senses of man be ministered 
to in order that there be no interferences in the central nerv- 
ous system, on the one hand, and every encouragement to the 
latter to furnish the necessary nervous impulses to the digest- 
ive organs and the tissues in every part of the organism : for 
it is not enough that food be digested in the ordinary sense ; 
it must also be built up into the tissues, a process depending, 
as we shall endeavor to show later, on the nervous system. 

The " gastrohoniiic art " has, therefore, become of great im- 


portance. It is as yet more of an art than a science ; the cook 
has outstripped the physiologist, if not the chemist also, in this 

"We can not explain fully why food prepared by certain 
methods and served in courses of a certain established order is 
so suited to refined man. A part is known, but a great deal 
remains to be discovered. We may, however, notice a few 
points of importance in regard to the preparation of food. ' 

It is now well established by experience that animals kept 
in confinement must have, in order to escape disease and attain 
the best results on the whole, a diet which not only imitates 
that of the corresponding wild forms generally, but even in 
details, with, it may be, altered proportions or added constitu- 
ents, in consequence of the difference in the environment. To 
illustrate : poultry can not be kept healthy confined in a shed 
without sand, gravel, old mortar, or some similar preparation ; 
and for the best results they must have green food also, as 
lettuce, cabbage, chopped green clover, grass, etc. They must 
not be provided with as much food as if they had the exercise 
afforded by running hither and thither over a large field. We 
have chosen this case because it is not commonly recognized 
that our domesticated birds have been so modified that special 
study must be made of the environment in all cases if they 
are not to degenerate. The facts in regard to homed cattle, 
horses, and dogs are perhaps better known. 

But all these instances are simple as compared with man. 
The lower mammals can live and flourish with comparatively 
little change of diet ; not so man. He demands diet not only 
dissimilar in its actual grosser nature, but differently prepared. 
In a word, for the efferent nervous impulses, on which the 
digestive processes depend to be properly supplied, it has be- 
come necessary that a variety of afferent impulses (through 
eye, ear, nose, palate) reach the nervous centers, attuning them 
to harmony, so that they shall act, yet not interfere with one 

Cooking greatly alters the chemical composition, the me- 
chanical condition, and, in consequence, the flavor, the digesti- 
bility, and the nutritive value of foods. To illustrate : meat in 
its raw condition would present mechanical difficulties, the di- 
gestive fluids permeating it less completely ; an obstacle, how- 
ever, of far greater magnitude in the case of most vegetable 
foods. By cooking, certain chemical compounds are replaced 
by others, while some may be wholly removed. As a rule. 


boiling is not a good form of preparing meat, because it with- 
draws not only salts of importance, but proteids and the ex- 
tractives — nitrogenous and other. Beef-tea is valuable chiefly 
because of these extractives, though it also contains a little 
gelatine, albumin, and fats. Salt meat furnishes less nutri- 
ment, a large part having been removed by the brine; not- 
withstanding, all persons at times, and some frequently, find 
such food highly beneficial, the effect being doubtless not con- 
fined to the alimentary tract. 

Meat, according to the heat employed, may be so cooked as 
to retain the greater part of its juices within it or the reverse. 
With a high temperature (65° to 70° C.) the outside in roasting 
may be so quickly hardened as to retain the juices. 

In feeding dogs it is both physiological and economical to 
give the animal the broth as well as the meat itself. The poor 
man may get excellent food cheaply by using not alone the 
meat of the shank of beef, but the extractives derived from it. 
There is much waste not only by the consumption of more food 
than is necessary, but by the purchase of kinds in which that 
important class of bodies, the proteids, comes at too high a 

It is remarkable in the highest degree that man's appetite, 
or the instinctive choice of food, has proved wiser than our 
science. It would be impossible even yet to match, by calcula- 
tions based on any data we can obtain, a diet for each man equal 
upon the whole to what his instincts prompt. With the lower 
mammals we can prescribe with greater success. At the same 
time chemical and physiological science can lay down general 
principles based on actual experience, which may serve to cor- 
rect some artificialities acquired by perseverance in habits that 
were not based on the true instincts of a sound body and a 
healthy mental and moral nature; for the influence of the 
latter can not be safely ignored even in such discussions as the 
present. These remarks, however, are meant to be suggestive 
rather than exhaustive. 

We may with advantage inquire into the nature of hunger 
and thirst. These, as we know, are safe guides usually in eat- 
ing and drinking. 

After a long walk on a warm day one feels thirsty, the 
mouth is usually dry; at all events, moistening the mouth, 
especially the back of it (pharynx), will of itself partially re- 
lieve thirst. But if we remain quiet for a little time the thirst 
grows less, even if no fluid be taken. The dryness has been 


relieved by the natural secretions. If, however, fluid be intro- 
duced into the blood either directly or through the alimentary 
canal, the thirst is also relieved speedily. The fact that we 
know when to stop drinking water shows of itself that there 
must be local sensations that guide us, for it is not possible to 
believe that the whole of the fluid taken can at once have en- 
tered the blood. 

Again, in the case of hunger, the introduction of innutritious 
matters, as earth or sawdust, will somewhat relieve the urgent 
sensations in extreme cases ; as will also the use of tobacco by 
smokers, or much mental occupation, though the latter is 
rather illustrative of the lessening of the consciousness of the 
ingoing impulses by diverting the attention from them. But 
hunger, like thirst, may be mitigated by injections into the 
intestines or the blood. It is, therefore, clear that, while in the 
case of hunger and thirst there is a local expression of a need, 
a peculiar sensation, more pronounced in certain parts (the 
fauces in the case of thirst, the stomach in that of hunger), 
yet these may be appeased from within through the medium 
of the blood, as well as from without by the contact of food or 
water, as the case may be. 

Up to the present we have assumed that the changes 
wrought in the food in the alimentary tract were identical 
with those produced by the digestive ferments as obtained by 
extracts of the organs naturally producing them. But for 
many reasons it seems probable that artificial digestion can not 
be regarded as parallel with the natural processes except in a 
very general way. When we take into account the absence of 
muscular movements, regulated according to no rigid prin- 
ciples, but varying with innumerable circumstances in all 
probability ; the absence of the influence of the nervous sys- 
tem determining the variations in the quantity and compo- 
sition of the outflow of the secretions ; the changes in the rate 
of so-called absorption, which doubtless influences also the act 
of the secretion of the juices — by these and a host of other con- 
siderations we are lead to hesitate before we commit ourselves 
too unreservedly to the belief that the processes of natural 
digestion can be exactly imitated in the laboratory. 

What is it which enables one man to digest habitually what 
may be almost a poison to another ? How is it that each one 
can dispose readily of a food at one time that at another is quite 
indigestible ? To reply that, in the one case, the digestive 
fluids are poured out and in the other not, is to go little below 


the surface, for one asks the reason of this, if it be a fact, as it 
no doubt is. When we look further into the peculiarities of 
digestion, etc., we recognize the influence of race as such, and 
in the race and the individual that obtrusive though ill-under- 
stood fact — the force of habit, operative here as elsewhere. 
And there can be little doubt that the habits of a people, as to 
food eaten and digestive peculiarities established, become or- 
ganized, fixed, and transmitted to posterity. 

It is probably in this way that, in the course of the evolu- 
tion of the various groups of animals, they have come to vary 
so much in their choice of diet and in their digestive processes, 
did we but know them thoroughly as they are ; for to assume 
that even the digestion of mammals can be summed up in the 
simple way now prevalent seems to us too broad an assump- 
tion. The field is very wide, and as yet but little explored. 

Human Physiology. — The study of Alexis St. Martin has fur- 
nished probably the best example of genuine human physiology 
to be found, and has yielded a harvest rich in results. 

We suggest to the student that self -observation, without 
interfering with the natural processes, may lead to valuable 
knowledge ; for, though it may lack some of the precision of 
laboratory experiments, it will prove in many respects more 
instructive, suggestive, and impressive, and have a bearing on 
medical practice that will make it telling. Not that we would 
be understood now or at any time as depreciating laboratory 
experiments ; but we wish to point out from time to time how 
much may be learned in ways that are simple, inexpensive, 
and consume but little time. 

The law of rhythm is illustrated, both in health and disease, 
in striking ways in the digestive tract. An individual long 
accustomed to eat at a certain hour of the day will experience 
at that time not only hunger, but other sensations, probably 
referable to secretion of a certain quantity of the digestive 
juices and to the movements that usually accompany the pres- 
ence of food in the alimentary tract. Some persons find their 
digestion disordered by a change in the hours of meals. 

It is well known that defecation at periods fixed, even within 
a few minutes, has become an established habit with hosts of 
people ; and the same is to a degree true of dogs, etc., kept in 
confinement, that are taught cleanly habits, and encouraged 
therein by regular attention to their needs. 

Now and then a case of what is very similar to regurgita- 
tion of food in ruminants is to be found among human beings. 


This is traceable to habit, which is bound up with the law of 
rhythm or periodic increased and diminished activity. 

Indeed, every one sufficiently observant may notice in him- 
self instances of the application of this law in the economy of 
his own digestive organs. 

This tendency is important in preserving energy for higher 
ends, for such is the result of the operation of this law every- 

The law of correlation, or mutual dependence, is well illus- 
trated in the series of organs composing the alimentary tract. 

The condition of the stomach has its counterpart in the rest 
of the tract : thus, when St. Martin had a disordered stomach, 
the epithelium of his tongue showed corresponding changes. 

We have already referred to the fact that one part may do 
extra work to make up for the deficiencies of another. 

It is confidently asserted of late that, in the case of persons 
long unable to take food by the mouth, nutritive substances 
given by enemata find their way up to the duodenum by anti- 
peristalsis. Here, then, is an example of an acquired adaptive 
arrangement under the stress of circumstances. 

It can not be too much impressed on the mind that in the 
complicated body of the mammal the work of any one organ 
is constantly varying with the changes elsewhere. It is this 
mutual dependence and adaptation — an old doctrine, too much 
left out of sight in modern physiology — which makes the at- 
tempt to completely unravel vital processes well-nigh hopeless ; 
though each accumulating true observation gives a better in- 
sight into this kaleidoscopic mechanism. 

We have not attempted to make any statements as to the 
quantity of the various secretions discharged. This is large, 
doubtless, but much is probably reabsorbed, either altered or 
unaltered, and used over again. In the case of fistulcB the con- 
ditions are so unnatural that any conclusions as to the normal 
quantity from the data they afford must be highly unsatisfac- 
tory. Moreover, the quantity must be very variable, accord- 
ing to the law we are now considering. It is well known that 
dry food provokes a more abundant discharge of saliva, and 
this is doubtless but one example of many other relations be- 
tween the character of the food and the quantity of secretion 

Evolution. — We have from time to time either distinctly 
pointed out or hinted at the evolutionary implications of the 
facts of this department of physiology. The structure of the 


digestive organs, plainly indicating a rising scale of complexity 
with greater and greater diflEerentiation of function, is, beyond 
question, an evidence of evolution. 

The law of natural selection and the law of adaptation, 
giving rise to new forms, have both operated, we may believe, 
from what can be observed going on around us and in our- 
selves. The occurrence of transitional forms, as in the epi- 
thelium of the digestive tract of the frog, is also in harmony 
with the conception of a progressive evolution of structure 
and function. But the limits of space will not permit of the 
enumeration of details. 

Summary. — A very brief resume of the subject of digestion 
will probably suffice. 

Food is either organic or inorganic and comprises proteids, 
fats, carbohydrates, salts, and water ; and each of these must 
enter into the diet of all known animals. They must also be 
in a form that is digestible. Digestion is the reduction of food 
to a form such that it may be further dealt with by the aliment- 
ary tract prior to being introduced into the blood (absorption). 
This is effected in different parts of the tract, the various con- 
stituents of food being differently modified, according to the 
secretions there provided, etc. The digestive juices contain 
essentially ferments which act only under definite conditions of 
chemical reaction, temperature, etc. 

The changes wrought in the food are the following : starches 
are converted into sugars, proteids into peptones, and fats into 
fatty acids, soaps, and emulsion ; which alterations are effected 
by-ptyalin and amylopsin, pepsin and trypsin, and bile and 
pa'ncreatic steapsin, respectively. 

Outside the mucous membrane containing the glands are 
muscular coats, serving to bring about the movements of the 
food along the digestive tract and to expel the fseces, the circu- 
lar fibers being the more important. These movements and the 
processes of secretion and so-called absorption are under the 
control of the nervous system. 

The preparation of the digestive secretions involves a series 
of changes in the epithelial cells concerned, which can be dis- 
tinctly traced, and take place in response to nervous stimula- 

These we regard as inseparably bound up with the healthy 
life of the cell. To be natural, it must secrete. 

The blood-vessels of the stomach and intestine and the villi 
of the latter receive the digested food for further elaboration 



(absorption). The undigested remnant of food and the excre- 
tions of the intestine make np the faeces, the latter being ex- 
pelled by a series of co-ordinated muscular movements essen- 
tially reflex in origin. 


In the mammal the breathing organs are lodged in a closed 
cavity, separated by a muscular partition from that in which 
the digestive and certain other organs are contained. This 

Pig. 295. — Lungs, anterior view (Sappey). 1 , upper lobe of left lung ; 2, lower lobe ; 3. fissure ; 
4, notch corresponding to apex of heart ; 5, pericardium ; 6, upper lobe of right lun^ ; 7. 
middle lobe ; 8, lower lobe ; 9. fissure : 10, fissure ; 11, diaphragm ; 12, anterior mediasti- 
num ; 13, thyroid gland : 14, middle cervical aponeurosis ; 15. process of attachment of 
mediastinum to pericardium ; 16, 16, seventh ribs ; 17, 17, transversales muscles ; 18, linea 



thoracic chamber may be said to be reserved for circulatory 
and respiratory organs which, we again point out, are so related 
that they really form parts of one system. 

The mammal's blood reqiiires so much aeration (ventilation) 
that the lungs are very large and the respiratory system has 
become greatly specialized. We no longer find the skin or ali- 
mentary canal taking any large share in the process ; and the 
lungs and the mechanisms by which they are made to move the 
gases with which the blood and tissues are concerned become 
very complicated. 

Fig. 296.— Bronchia and lungs, posterior view (Sappey). 1. 1. summit of lungs ; 2, 2, base of 
lungs ; 3, trachea ; 4, right bronchus ; 5, division to upper lobe of lung ; 6, division to 
lower lobe ; 7, left bronchus ; 8. division to upper lobe ; 9, division to lower lobe ; 10. left 
branch of pulmonary artery ; 11, right branch ; 12, left auricle of heart ; 13, left superior 
pulmonary vein ; 14, left inferior pulmonary vein ; 15, right superior pulmonary vein : 16, 
right inferior pulmonary vein ; 17. inferior vena cava ; 18, left ventricle of heart ; 19, right 

Our studies of muscle physiology should have made clear 
the fact that tissue-life implies the constant consumption of 
oxygen and discharge of carbonic anhydride, and that the pro- 
cesses which give rise to this are going on at a rapid rate ; so 
that the demands of the animal for oxygen constantly may be 
readily understood if one assumes, what can be shown, though 
less readily than in the case of muscle, that all the tissues are 
constantly craving, as it were, for this essential oxygen — well 
called " vital air." 



Respiration may, then, be regarded from a physical and 
chemical point of view, though in this as in other instances we 

Fig. 297. — Trachea and branchial tubes (Sappey). 1, 2, larynx ; -S, 3, trachea ; 4, bifurcation 
of trachea ; 5, right bronchus ; G, left bronchus ; 7, bronchial division to upper lobe of 
right lung ; 8. division to middle lobe ; 9, division to lower lobe ; 10. division to upper lobe 
of left lung ; 11, division to lower lobe ; 13, 12, 12, 12, ultimate ramifications of bronchia ; 
13, 13, 13, 13, lungs, represented in contour ; 14, 14, summit of lungs ; 15, 15, base of lungs. 

must be on our guard against regarding physiological processes 
as ever purely physical or purely chemical. The respiratory 
process in the mammal, unlike the frog, consists of an active 
and a (largely) passive phase. The air is not pumped into the 
lungs, but sucked in. So great is the complexity of the lungs 
in the mammal, that the frog's lung (which may be readily 
understood by blowing it up by inserting a small pipe in the 
glottic opening of the animal and then ligaturing the distended 
organ) may be compared to a single infundibulum of the mam- 
malian lung. 

Assuming that the student is somewhat conversant with the 



coarse and fine anatomy of the respiratory organs, we call at- 
tention to the physiological aspects of some points. The lungs 
represent a membranous expansion of great extent, lined with 
flattened cells and supporting innumerable capillary blood-ves- 
sels. The air is admitted to the complicated foldings of this 
membrane by tubes which remain, throughout the greater part 
of their extent, open, being composed of cartilaginous rings, 
completed by soft tissues, of which plain muscle-cells form an 

Fia. 298.— Mold of a terminal bronchus and a group of air-cells moderately distended by- 
injection, from the human subject (Robin). 

important part, serving to maintain a tonic resistance against 
pulmonary and bronchial pressure, as well as serving to aid 
in the act of coughing, etc., so important in expelling foreign 
bodies or preventing their ingress. 

The bronchial tubes are lined with a mucous membrane, 
kept moist by the secretions of its glands, and covered with 
ciliated epithelium, as are also the nasal passages, which by 
the outward currents they create, favor diffusion of gases, and 
removal of excess of mucus. The thoracic walls and the lungs 
themselves are covered with a tough but thin membrane lined 
with flattened cells, which secrete a small quantity of fluid, 



that serves to maintain the surrounding parts in a moist con- 
dition, thus lessening friction. The importance of this ar- 

'^^s'-^'X*^ -&- ^^'^ 



Fig. 299.— Section of the parenchyma of the human lung, injected through the pulmonary- 
artery (Schulze). a, a, c, c, walls of the air-cells ; 6, small arterial branch. 

rangement is well seen when, in consequence of inflammation 
of this pleura, it becomes dry, giving rise during each resjjira- 
tory movement to a friction-sound and a painful sensation. 
It will not be forgotten that this membrane extends over the 
diaphragm, and that, in consequence of the lungs completely 
filling all the space (not occupied by other organs) during every 
position of the chest- walls, the costal and pulmonary pleural 
surfaces are in constant contact. By far the greater part of 
the lung-substance consists of elastic tissue, thus adapting the 
principal respiratory organs to that amount of distention and 
recoil to which they are ceaselessly subjected during the en- 
tire lifetime of the animal. 

The Entrance and Exit of Air. 

Since the lungs fill up so completely the thoracic cavity, 

manifestly any change in the size of the latter must lead to 

an increase or diminution in the quantity of air they contain. 

Since the air within the respiratory organs is being constantly 




robbed of its oxygen, and rendered impure by the addition of 
carbonic dioxide, the former must be renewed and the latter 

expelled ; and, as mere diffu- 
sion takes place too slowly to 
accomplish this in the mam- 
mal, this process is assisted 
by the nervous system set- 
ting certain muscles at work 
to alter the size of the chest 
cavity. Because of the ribs 
being placed obliquely, it fol- 
lows that their elevation will 
result in the enlargement of 
the thoracic cavity in the an- 
tero-posterior diameter ; and, 
as the chest, in consequence, 
gets wider from above down- 
ward, also in the transverse 
diameter ; which is more- 
over assisted by the eversion 
of the lower borders of the 
ribs ; and, if the convexity of 
the diaphragm were dimin- 
ished by its contraction and 
consequent descent, it would follow that the chest would be in- 
creased in the vertical diameter also. All these events, favor- 
able to the entrance of air, actually take place through agencies 
we must now consider. The student is recommended to look 
into the insertion, etc., of the muscles concerned, to which we 
can only briefly refer. 

The act of inspiration commences by the fixation of the 
uppermost ribs, beginning with the first two, by means of the 
scaleni muscles, this act being followed up by the contraction 
of the external intercostals, leading to the elevation of the 
other ribs ; at the same time, the arch of the diaphragm de- 
scends in consequence of the contraction of its various mus- 
cular bundles. Under these circumstances, the air from Avith- 
out must rush in, or a vacuum be formed in the thoracic 
cavity ; and, since there is free access for the air through the 
glottic opening, the lungs are of necessity expanded. This in- 
going air has had to overcome the elastic resistance of the 
lungs, which amounts to about 5 millimetres of mercury in 
man, as ascertained by tying a manometer in the windpipe of 

Fig. 300.— Diag:ram illustrating: elevation of ribs 
in inspiration (Bi5clard). The dark lines rep- 
resent the ribs, sternum, and costal cartilages 
in inspiration. 



a dead subject, and then opening the thorax to equalize the 
inside and outside pressures, when the lungs at once collapse 
and the manometer shows a 
rise of the mercury to the ex- 
tent indicated above. To this 
we must add the influence of 
the tonic contraction of the 
bronchial muscles before re- 
ferred to, though this is prob- 
ably not very great. 

That there are variations 
of intrapulmonary pressure 
may be ascertained by con- 
necting a manometer with one 
nostril — the other being closed 
— or with the windpipe. The 
mercury shows a negative 
pressure with each inspirato- 
ry, and a positive with each 
expiratory act. This may 
amount to from .30 to 70 mil- 
limetres with strong inspira- 
tion, and 60 to 100 in forcible 

When inspiration ceases, the elastic recoil of the rib carti- 
lages and the ribs themselves, and of the sternum, the weight 

- Diagrammatic representation of 
action of diaphragm in inspiration (Her- 
mann), Vertical section through second 
rib on right side. The broken and dotted 
lines show the amount of the descent of the 
diaphragm in ordinary and in deep Inspira- 

Fig. 302.— Apparatus to illustrate relations of intra-thoracic and external pressiu-es {after 
Beaunis). A glass bell-jar is provided with a light stopper, through which passes a branch- 
ing glass tube fitted with a pair of elastic bags representing lungs. The bottom of the jar is 
closed by rubber membrane representing diaphragm. A mercury manometer indicates 
the difference in pressure within and without the bell- jar. In left-hand figure it will be 
seen that these pressures are equal ; in right (inspiration), the external pressure is consid- 
erably greater. At one part (6) an elastic membrane fills a hole in jar, representing an 
intercostal space. 



Fig. 303.— Dorsal view of four vertebrse 
and three attached ribs, showing 
attachment of elevator muscles of 
ribs and intercostals (after Allen 
Thomson). 1, long and short eleva- 
tors ; 2, external intercostal ; 3, in- 
ternal intercostal. 

of these parts and that of the attached muscles, etc., assists in 
the return of the chest to its original position, entirely indepen- 
dently of the action of muscles. 
Moreover, with the descent of the 
diaphragm the abdominal viscera 
have been thrust down and com- 
pressed together with their included 
gases ; when this muscle relaxes, 
they naturally exert an upward 
pressure. Putting these events 
together, it is not difiScult to un- 
derstand why the air should be 
squeezed out of the lungs, the elas- 
ticity of which latter is, as we have 
shown, an important factor in itself. 
The Muscles of Respiration. — The 
diaphragm may be considered the 
most important single respiratory 
muscle, and can of itself maintain 
respiration. The scaleni are important as fixators of the ribs ; 
the levatores costarum, and external intercostals, as normal ele- 
vators. The quadratus lumborum 
assists the diaphragm by fixing the 
last rib. These, with the serratus 
porticus superior, may be regarded 
as the principal muscles called into 
action in an ordinary inspiration. 
The muscles used in an ordinary ex- 
piratory act are the internal intercos- 
tals, the triangidaris sterni, and ser- 
ratus posticus inferior. In forced 
inspiration the lower ribs are drawn 
down and retracted, giving support 
in their fixed position to the dia- 
phragm. The scaleni, pectorales, 
serratus magnus, latissimus dorsi, 
and others are called into action ; but 
when dyspnoea becomes extreme, as 
in one with a fit of asthma, nearly all 
the muscles of the body may be called 
into play, even the muscles of the 
face, which are not normally active at all or but very slightly 
in natural breathing. 

Fig. 304, 

Laryngoscopic views of 
the glottis, etc. (after Quain and 
Czermak). I. Larynx in quiet 
breathing. II. During a deep in- 
spiration. In this case the rings 
of the trachea and commence- 
ment of bronchi are visible. Such 
a condition is persistent in many 
forms of disease in which respir- 
ation is attended with difficu] 




Facial and laryngeal respiration is best seen in sucli animals 
as the rabbit, and it is this condition -which is approximated 
in disordered states in man — in fact, when from any cause in- 
spiration is very labored (asthma, diphtheria, etc.). 

In man and most mammals, unlike the frog, the glottic 
opening is never entirely closed during any part of the respira- 
tory act, though it undergoes a rhythmical change, of size, 
widening during inspiration and narrowing during expiration, 
in accordance with the action of the muscles attached to the 
arytenoid cartilages, the action of which may be studied in 
man by means of the laryngoscope. 

The abdominal muscles have a powerful rhythmical action 
during forced respiration, though whether they function dur- 
ing ordinary quiet breathing is undetermined; if at all, prob- 
ably but slightly. Though the removal of the external inter- 
costals in the dog and some other animals reveals the fact that 
the internal intercostals contract alternately with the dia- 
phragm, it must not be regarded as absolutely certain that such 
is their action when their companion muscles are present, for 
Nature has more ways than one of accomplishing the same pur- 
pose — a fact that seems often to be forgotten in reasoning from 
experiments. This result, however, carries some weight with it. 

Types of Bespiration. — There are among mammals two princi- 
pal types of breathing recognizable — the costal (thoracic) and 
abdominal — according as the movements of the chest or the 
abdomen are the more pronounced. 

In the civilized white woman, even in the female child, the 
upper thorax takes a larger share in respiration than in the 
male sex. This has been explained, on the one hand, as being 
due to artificial influences, modes of dress, and their inherited 
effects ; and on the other to natural ones, the crowding of the 
respiratory organs, owing to the contents of the pelvic and 
abdominal cavities encroaching on the thorax, in consequence 
of the enlargement of the uterus during pregnancy. It has, 
however, been maintained recently that an examination of 
pure-blooded Indian girls does not show the features of respira- 
tion just noticed as characteristic of the breathing of white 
fefiiales, the inference from which is obvious. But, again, it is 
to be remembered that the Indian and other women retaining 
primitive habits possess a power of adaptation to the demands 
of the pregnant condition no longer shown by white women. 
Thoracic breathing in females is probably the result of several 
co-operating causes, of which usage in dress is one. 



Personal Observation. — The student would do well at this stage 
to test the statements we have made in regard to the respira- 
tory movements on the human subject especially. This he 
can very well do in his own person when stripped to the waist 
before a mirror. Many of the abnormalities of the forced res- 
piration of disease may be imitated — in fact, this is one of the 
departments of physiology in which the human aspects may 
be examined into by a species of experiment on one's self that 
is as simple as it is valuable. 

Fig. 305. — Protula dysteri, a marine annelid living in a calcareous tube constructed by itself 
(after Huxley). The cut represents the sexualljy mature animal (hermaphrodite) extracted 
from its calcareous tube, a, branchial (respiratory) plumes, abundantly vascular ; b, 
hood-like expansion of anterior end of body ; c, mouth ; d, stomach ; e, anus ; /, testes ; 
5, ova. 



Fig. 306. 

Fig. 307, 

Fig. 306. —Vertical transverse section of fresh-water mussel (Anodon) through heart (after 

Huxley). V, ventricle ; a. auricles ; ?•, rectum ; p, pericardium ; t, inner, o, outer gill ; 

o', vestibule of organ of Bojanus, B ; /, foot ; m, m, mantle lobes. 
Fig. 307.— Gill of fish (perch), to illustrate relations of different blood-vessels, etc., concerned 

in respiration (after Bell). A, branchial artery ; B, branchial arch seen in cross-section ; 

V, branchial vein ; a, v, branches of artery and vein respectively. 

Comparative. — It is hoped that the various figures accompa- 
nied by descriptions, introduced in this and other chapters, will 
make the relations of the circulation and respiration in the va- 

vi viivmix y 

Fig. 308.— Diagram of scorpion, most of the appendages having been removed (after Huxley). 
o, mouth ; 6, alimentary tract ; c, anus ; d, heart ; e, pulmonary sac ; /, position of ven- 
tral ganglionated cord ; g, cerebral ganglia ; T, t^lson. VII— XX, seventh to twentieth 
somite. IV, V, VI, basal joints of pedipalpi and two following pairs of limbs. 



rious classes of animals, whether terrestrial or aquatic, evident 
"without extended treatment of the subject in the text. What 


Fig. 309. 

Fig. 309. — A. Pulmonary sac. B. respiratory leaflets of Scorpio occifanus fafter Blanchard). 
Fig. 310. — Left pulmonary sac, viewed from dorsal aspect, of a spider (after Dug^s). pm, 
pulmonary lamellae ; sig^ stigma, or opening to former. 

we are desirous of impressing is that throughout the entire ani- 
mal kingdom respiration is essentially the same process ; that 

Fig. 311.- -A. B. Tadpoles with external branchife (after Huxley) m, nasal sacs ; a, eye ; o, 
ear ; k. 6, branchige ; m, mouth ; z, horny jaws ; s, suckers ; rf, opercular tor gill) fold. 
C. More advanced frog's larva, y, rudiment of hind-limb ; k. s, single branchial aperture. 
Owing to figure not having been reversed, this aperture seems to he on right instead of 
left side. 

finally it resolves itself into tissue-breathing: the appropria- 
tion of oxj'gen and the excretion of carbon dioxide. Since the 
manner in which oxygen is introduced into the lungs and foul 
gases expelled from them in some reptiles and amphibians, is 
largely different from the method of respiration in the mam- 
mal, we call attention to this process in an animal readily 
watched — the common frog. This creature, by depressing the 
floor of the mouth, enlarges his air-space in this region and 
consequently the air freely enters through the nostrils ; where- 
upon the latter are closed by a sort of valve, the glottis opened 



and the air forced into the lungs by the elevation of the floor 
of the mouth. By a series of flank movements the elasticity 
of the lungs is aided in expelling the air through the now open 
nostrils. The respiration of the turtle and some other reptiles 
is somewhat similar. In the case of aquatic animals, both in- 

Fia. 312.— General view of air-reservoirs of duck, opened inferiorly ; also their relations with 
principal viscera of trunlc (after Sappey). 1, 1, anterior extremity of cervical reservoirs ; 
3, thoracic reservoir ; 3, anterior diaphragmatic reservoir ; 4, posterior ditto ; 6, abdom- 
inal reservoir : a, membrane forming anterior diaphragmatic reservoir ; 6, membrane 
forming posterier ditto ; 6, section of thoraeo-abdominal diaphragm : d, subpectoral pro- 
longation of thoracic reservoir ; e, pericardium ; /, /, liver ; (?, gizzard ; ft, intestines ; m, 
heart ; ?i, 7i, section of great pectoral muscle above its insertion into the htnnerus ; o, an- 
terior clavicle ; p^ posterior clavicle of right side cut and turned outward. 

vertebrate and vertebrate, excepting mammals, the blood is 
freely exposed in the gills to oxygen dissolved in the water as 
it is to the same gas mixed with nitrogen in terrestrial animals. 
In the land-snail, land-crab, etc., we have a sort of intermedi- 
ate condition, the gills being kept moist. It is not to be for- 


gotten, however, that normally the respiratory tract of mam- 
mals is never other than slightly moist. 

The Quantity of Air respired. 

We distinguish between the quantity of air that usually is 
moved hy the thorax, and that which may be respired under 
special effort, which, of course, can never exceed the capacity 
of the respiratory organs. 

Accordingly, we recognize: 1. Tidal air, or that which 
passes in and out of the respiratory passages in ordinary quiet 
breathing, amounting to about 500 cc, or thirty cubic inches. 
2. Complemental air, which may be voluntarily inhaled by a 
forced inspiration in addition to the tidal air, amounting to 
1,500 cc, or about 100 cubic inches. 3. Supplemental {reserve) 
air, which may be expelled at the end of a normal respiration 
— i. e., after the expulsion of the tidal air, and which represents 
the quantity usually left in the lungs after a normal quiet 
expiration, amounting to 1,500 cc. 4. Residual air, which can 
not be voluntarily expelled at all, amounting to about 2,000 cc, 
or 120 cubic inches. 

The vital capacity is estimated by the quantity of air that 
may be expired after the most forcible inspiration. This will, 
of course, vary with the age, which determines largely the elas- 
ticity of the thorax, together with sex, position, height, and a 
variety of other circumstances. But, inasmuch as the result 
may be greatly modified by practice, like the power to expand 
the chest, the vital capacity is not so valuable an indication as 
might at first be supposed. 

It is important to bear in mind that the tidal air is scarcely 
more than sufficient to fill the upper air-passages and larger 
bronchi, so that it requires from five to ten respirations to re- 
move a quantity of air inspired by an ordinary act. Very 
much must, therefore, depend on diffusion, the quantity of air 
remaining in the lungs after each breath being the sum of the 
residual and reserve air, or about 3,500 cc. (220 cubic inches). 
Considering the creeping slowness of the capillary circulation, 
it would not be supposed that the respiratory process in its 
essential parts should be the rapid one that a greater move- 
ment of the air would imply. 


The Respiratory Rhythm. 

In man, and most of our domestic mammals, a definite rela- 
tion between the cardiac and respiratory movements obtains, 
tbere being about four to five heart-beats to one respiration, 
which would make the rate of breathing in man about sixteen 
to eighteen per minute. Usually, of course, the largest animals 
have the slower pulse and respiration ; and this is an invariable 
rule for the varieties of a species, as observable in the canine 
race, to mention a well-known instance. 

The rate of the respiratory movements is to some extent a 
measure of the rapidity of the oxidative processes in the body, 
as witness the slow and intermittent breathing of cold-blooded 
animals as compared with the more rapid respiration of birds 
and mammals (Fig. 313). 

Pathological. — Any condition that lessens the amount of re- 
spiratory surface, or diminishes the mobility of the chest-walls 
is usually accompanied by accelerated movements, but beneath 
this is the demand for oxygen, part of the avenues by which 
this gas usually enters, having been closed or obstructed by the 
disease. So that it is not surprising that, in consequence of 
the effusion of fluid into the thoracic cavity, leading to the 
compression of the lung, the opposite one should be called into 
more frequent use, and even enlarge to meet the demand. 
These facts show how urgent is the need for constant ventila- 
tion of the blood, and at the same time how great is the power 
of adaptation to meet the emergency. 

The difference between the inspiratory and the expiratory 
rhythm may be gathered by watching the movements of the 
bared chest, or more accurately from a graphic record. It is 
usually considered that expiration is only slightly longer than 
inspiration, and that any marked deviation from this relation 
should arouse suspicion of disease. Normally the respiratory 
pause is very slight, so that inspiration seems -to follow di- 
rectly on expiration ; though the latter act reminds us of the 
prolongation of the ventricular systole after the blood is ex- 

If, in the tracing, the small waves on the upper part of the 
expiratory curve really represent the effect of the heart-beat, it 
makes it easier to understand how such might assist in venti- 
lating the blood when the respirations occur only once in a 
considerable interval and very feebly then, as in hibernating 
animals or individuals that have fainted ; though it must be 



remembered tliat difEusion is a ceaseless process in all living 

It is scarcely necessary to point out that the respiratory 



Fio. 313.— Tracings of respiratory movements of individuals belonging to different groups pt 
the animal Icingdom (after TSianhoffer). Differences in depth, frequency, and «»Pf f'^/ 
regularity, are very noticeable. 1, flsh; 2, tortoise; 3, adder (in wmter); 4, boa^con- 
stflctor (m summer); B, frog; 6, alligator; 7, lizard; 8, canary-bird; 9, adult dog ,10, 
rabbit ; 11, man ; 13, dog ; IS, horse. Compare these, and note that in nl respiration is 
shallow, and in ml deep. 

movements are increased by exercise, emotions, position, sea- 
son, hour of the day, taking meals, etc. 



Fia. 314. — Tracings of respiration of horse wlien at rest and after exercise (after Thanhoffer). 
/, inspiration ; E, expiration. Spaces between vertical lines indicate time periods of one 
second each. 1, animal standing at rest ; 3, after .walk of few minutes ; 7 and' 8, after 
trotting ; 9, after a brief rest ; 11, after trotting and running for some minutes ; 17, after 
resting from last for a short time ; SI, tracing at end of experiment. 

Respiratory Sounds. — The entrance and exit of air are accom- 
panied by certain sounds, which vary with each part of the 
respiratory tract To these sounds names have been given, but 
as they are somewhat inconstant in their application, or at least 
have several synonyms, we pass them by, recommending the 
student to actually learn the nature of the respiratory murmurs 
by listening to the normal chest in both man and the lower ani- 
mals. With the use of a double stethoscope he may practice upon 
himself, though not so advantageously as in the case of the heart. 

The sounds are caused in part by the friction of the air, 
though they are probably complex, several factors entering into 
their causation. 

Comparison op the Inspired and Expired Air. 

The changes that take place in the air respired may be 
briefly stated as follows : 


1. Whatever tte condition of the inspired air, that expired 
is about saturated with aqueous vapor — i. e., it contains all that 
it is capable of holding at the existing temperature. 

2. The temperature of the expired air is about that of the 
blood itself, so that if the air is very cold when breathed, the 
body loses a great deal of its heat in warming it. The expired, 
air of the nasal passages is slightly warmer than that of the 

3. Experiment shows that the expired air is really dimin- 
ished in volume to the extent of from one fortieth to one fiftieth 
of the whole. Since two volumes of carbonic anhydride require 
for their composition two volumes of oxygen, if the amount of 
the former gas expired be not equal to the amount of oxygen 
inspired, some of the latter must have been used to form other 


combinations. -^, amounting to rather less than 1, is called 

the respiratory coefficient, 

4. The difference between inspired and expired air may be 
gathered from the following : 

Inspired air 

Expired air 



Carbonic dioxide. 







From which the most important conclusions to be drawn 
are, that the expired air is poorer in oxygen to the extent of 
4 to 5 per cent, and richer in carbonic anhydride to somewhat 
less than this amount. 

From experiment it has been ascertained that the amount 
of carbonic dioxide is for the average man 800 grammes (406 
litres, equivalent to 318'1 grammes carbon) daily, the oxygen 
actually used for the same period being 700 grammes. But 
the variations in such cases are very great, so that these num- 
bers must not be interpreted too rigidly. Experience proves 
that, while chemists often work in laboratories in which the 
percentage of carbonic anhydride (from chemical decomposi- 
tions) reaches 5 per cent, an ordinary room in which the amount 
of this gas reaches 1 per cent is entirely unfit for occupation. 
This is not because of the amount of the carbon dioxide pres- 
ent, but of other impurities which seem to be excreted in pro- 
portion to the amount of this gas, so that the latter may be 
taken as a measure of these poisons. 

What these are is as yet almost entirely unknown, but that 
they are poisons is beyond doubt. Small effete particles of 


once-living protoplasm are carried out witli the breath, hut 
these other substances are got rid of from the blood by a vital 
process of secretion (excretion), we must believe ; which shows 
that the lungs to some degree play the part of glands, and that 
their whole action is not to be explained as if they were merely 
moistened bladders acting in accordance with ordinary physical 

An estimation of the amount of atmospheric air required 
may be calculated from data already given. 

Thus, assuming that a man gives up at each breath 4 per 
cent of carbon dioxide to the 500 cc. of tidal air he expires, and 
breathes, say, seventeen times a minute, we get for the amount 
of air thus charged in one hour to the extent of 1 per cent : 

500 X 4 X 17 X 60 = 3,040,000 cc, or 3,040 litres. 

But if the air is to be contaminated to the extent of only 
^ per cent of carbonic anhydride, the amount should equal at 
least 3,040 X 10 hourly. 

Respiration in the Blood. 

It may be noticed that arterial blood kept in a confined 
space grows gradually darker in color, and that the original 
bright scarlet hue may be restored by shaking it up with air. 
When the blood has passed through the capillaries and reached 
the veins, the color has changed to a sort of purple, character- 
istic of venous blood. Putting these two facts together, we are 
led to suspect that the change has been caused in some way by 
oxygen. Exact experiments with an appropriate form of blood- 
pump show that from one hundred volumes of blood, whether 
arterial or venous, about sixty volumes of gas may be obtained ; 
that this gas consists chiefly of oxygen and carbonic anhydride, 
but that the proportions of each present depends upon whether 
the blood is arterial or venous. 

The following table will make this clear : 

Arterial blood. 
Venous blood.. 


Carbonic anhydride. 








from 100 volumes of blood at 0° C. and 760 millimetre pressure. 

Arterial blood, then, contains 8 to 13 per cent more oxygen 

and about 6 per cent more carbonic dioxide than venous blood. 

It is not, of course, true, as is sometimes supposed, that arterial 



blood is " pure blood " in the sense that it contains no carbonic 
anhydride, as in reality it always carries a large percentage of 
this gas. 

Fia. 315, — Diagrammatic illustration of Ludwig's mercurial gas-pump. A and B are two 
glass globes, connected by strong India-rubber tubes, with two similar glass globes, 
A' and B'. A is further connected by means of the stop-cock c with the receiver C, 
containing the blood (or other fluid) to be analyzed ; and B^hj means of the stop-cock 
d and tube e with the receiver jD, for receiving the gases. A and JB are also connected 
with each other by means of the stop-cocks / and g^ the latter being, so arranged that B ' 
also communicate with B' by the passage g'. A' and B' being fuliof mercury, and the 
cocks fc, /, g and d being open, but c and g' closed, on raising A' by means of the pulley p 
the mercury of A' fills A, driving out the air contained in it into B, and so out through e. 
"When the mercury has risen above g, f is closed ; and g' being opened, B' is in turn raised 
till B is completely filled with mercury, all the air previously in it being driven out 
through e. Upon closing d and lowering B\ the whole of the mercury in £ falls into B', 
and a vacuum consequently is established in B. On closing g' but opening g, /» and fc, and 
lowering A\ a vacuum is similarly established in A and in the junction between A and B. 
If the cock c be now opened, the gases of the blood in C escape into the vacuum of A and 
B. By raising ^' after the closure of c and opening of d, the gases so set free eire driven 
from A into B, and by the raising of B' from B through e into the receiver D, standing 
over mercury. (After Foster.) 

The Conditions under which the Gases exist in the Blood. — If a 

flnidj as water, be exposed to a mixture of gases which it can 


absorb under pressure, it is found that the amount taken up 
depends on the quantity of the particular gas present independ- 
ent of the presence or quantity of the others ; thus, if water 
be exposed to a mixture of oxygen and nitrogen, the quantity 
of oxygen absorbed ■will be the same as if no nitrogen were 
present — i. e., the absorption of a gas varies with the partial 
pressure of that gas in the atmosphere to which it is exposed. 
But whether blood, deprived of its gases, be thus exposed to 
oxygen under pressure, or whether the' attempt be made to 
remove this gas from arterial blood, it is found that the above- 
stated law does not apply. 

When blood is placed under the exhaustion-pump, at first 
very little oxygen is given off ; then, when the pressure is con- 
siderably reduced, the gas is suddenly liberated in large quan- 
tity, and after this comparatively little. A precisely analogous 
course of events takes place when blood deprived of its oxygen 
is submitted to this gas under pressure. On the other hand, 
if these experiments be made with serum, absorption follows 
according to the law of pressures. Evidently, then, if the oxy- 
gen is merely dissolved in the blood, such solution is peculiar, 
and we shall presently see that this supposition is neither neces- 
sary nor reasonable. 


Haemoglobin constitutes about ^ of the corpuscles, and, 
. though amorphous in the living blood-cells, may be obtained 
in crystals, the form of which varies with the animal; in- 
deed, in many animals this substance crystallizes spontane- 
ously on the death of the red cells. It is unique among albu- 
minous compounds in being the only one found in the animal 
body that is susceptible of crystallization. Its estimated com- 
position is : 

Carbon 53-85 

Hydrogen 7-33 

Nitrogen 16-17 

Oxygen 21-84 

Iron -43 

Sulphur -39 

together with 3 to 4 per cent of water of crystallization. 

The formula assigned is : CeooHjeoOiTaNiMFeSs. The molecular 
constitution is not known, and the above formula is merely an 
approximation, which will, however, serve to convey an idea 




of the great complexity of this compound. The presence of 
iron seems to be of great importance. If not the essential 

respiratory constituent, cer- 
tainly the administration of 
this metal in some form 
proves very valuable when 
the blood is deficient in 

This - substance can be 
recognized most certainly by 
the spectroscope. The ap- 
pearances vary with the 
strength of the solution, 
and, as this test for blood 
(haemoglobin) is of much 
practical importance, it will 
be necessary to dwell a little 
upon the subject ; though, 
after a student has once rec- 
ognized clearly the differ- 
ences of the spectrum ap- 
pearances, he has a sort of 
knowledge that no verbal 
description can convey. This 
is easily acquired. One only 
needs a small, flat-sided bot- 
tle and a pocket - spectro- 
scope. Filling the bottle 
half-full of water, and getting the spectroscope so focused that 
the Fraunhofer lines appear distinctly, blood, blood-stained 
serum, a solution of haemoglobin-crystals, or the essential sub- 
stance in any form of dilute solution, may be added drop by 
drop till changes in the spectrum in the form of dark bands 
appear. By gradually increasing the quantity, appearances 
like those figured below may be observed, though, of course, 
much will depend on the thickness of the layer of fluid as to 
the quantity to be added before a. particular band comes into 

When wishing to be precise, we speak of the most highly 
oxidized form of haemoglobin as oxy-haemoglobin (0-H), and 
the reduced form as haemoglobin simply, or reduced haemo- 
globin (H). 

By a comparison of the spectra it will be seen that the bands 

Fig. 316. — Crystallized haemoglobin (Gautier). 
' ■ bl< " ' 

a, b, crystals from venous blood of man 
from blood of cat ; d, of Guinea-pig ; e, 
marmot ; /, of squirrel. 




of oxy-haemoglobia lie between the D and E lines ; that the 
left band near D is always the most definite in outline and the 

most pronounced in every respect except breadth ; that it is in 
weak solutions the first to appear, and the last to disappear on 


reduction ; that there are two instances in which there may be 
a single band from haemoglobin — in the one case when the solu- 
tion is very dilute and when it is very concentrated. These 
need never be mistaken for each other nor for the band of re- 
duced haemoglobin. The latter is a hazy broad band with com- 
paratively indistinct outlines, and darkest in the middle. 

It will be further noticed that in all these instances, apart 
from the bands, the spectrum is otherwise modified at .each 
end, so that the darker the more centrally placed characteristic 
bands, the more is the light at the same time cut off at each 
end of the spectrum. 

If, now, to a specimen showing the two bands of oxy -haemo- 
globin distinctly a few drops of ammonium sulphide or other 
reducing agent be added, a change in the color of the solution 
will result, and the single hazy band characteristic of haemo- 
globin will appear. ' 

It is not to be supposed, however, that venous blood gives 
this spectrum. Even after asphyxia it will be difficult to see 
this band, for usually some of the oxy -haemoglobin remains 
reduced ; but it is worthy of note, as showing that the appear- 
ances are normal, that the blood, viewed through thin tissues 
when actually circulating, whether arterial or venous, gives 
the spectrum of oxy-haemoglobin. At the same time there can 
be no doubt that the changes in color which the blood under- 
goes in passing through the capillaries is due chiefly to loss of 
oxygen, as evidenced by the experiments before referred to ; and 
the reason that the two bands are always to be seen in venous 
blood is simply that enough oxy-haemoglobin remains to give 
the two-band spectrum which prevails over that of (reduced) 
haemoglobin. We are thus led by many paths to the important 
conclusion that the red corpuscles are oxygen-carriers, and, 
though this may not be and probably is not their only func- 
tion, it is without doubt their principal one. Of their oxygen 
they are being constantly relieved by the tissues; hence the 
necessity of a circulation of the blood from a respiratory point 
of view. 

There are other gases that can replace oxygen and form 
compounds with haemoglobin ; hence we have CO-haemoglobin 
and NO-haemoglobin, which in turn are replaced by oxygen with 
no little difficulty — a fact which explains why carbonic oxide is 
so fatal when respired, and, as it is a constituent of illuminat- 
ing gas, the cause of the death of those inhaling the latter is 
often not far to seek. Blood may, in fact, be saturated with 


carbonic oxide by allowing illuminating gas to pass through it, 
when a change of color to a cherry red may be observed, and 
which will remain in spite of prolonged shaking up with air or 
attempts at reduction with the usual reagents. Haemoglobin' 
may be resolved into a proteid (globin) not well understood, 
and hcemaiin. This happens when the blood is boiled (perhaps 
also in certain cases of lightning-stroke), and when strong acids 
are added. Hsematin is soluble in dilute acids and alkalies, and 
has then characteristic spectra. Alkaline hsematin may be re- 
duced ; and, as the iron can be separated, resulting in a change 
of color to brownish red, after which there are no longer any 
reducing effects, it would seem that the oxygen-carrying power 
and iron are associated. This iron-free hsematin is named 
hcRmatoporphyrin or hcematoin. 

Hcemin is hydrochlorate of hsematin (Teichmann's crystals), 
and may be formed by adding glacial acetic acid and common 
salt to blood, dried blood-clot, etc., and heating to boiling. This 
is one of the best tests for blood, valuable in medico-legal and 
other cases. 

When oxy -haemoglobin stands exposed to the air, or when 
diffused in urine, it changes color and becomes, in fact, another 
substance — methcemoglobin, irreducible by other gases (CO, etc.), 
and not surrendering its oxygen in vacuo, though giving it up 
to ammonium sulphide, becoming again oxy-hsemoglobin, when 
shaken up with atmospheric air. Its spectrum differs from 
that of oxy-haemoglobin in that it has a band in the red end of 
the spectrum between the C and D lines. Hcematoidin is some- 
times found in the body as a remnant of old blood-clots. It is 
probably closely allied to if not identical with the bilirubin 
of bile. 

Comparative. — While haemoglobin is the respiratory agent in 
all the groups of vertebrates, this is not true of the inverte- 
brates. Red blood-cells have as yet been found in but a few 
species, though haemoglobin does exist in the blood plasma of 
several groups, to one of which the earth-worm and several- 
other annelids belong. It is interesting to note that the respir- 
atory compound in certain families of crustaceans, as the com- 
mon crab, horseshoe-crab (limulus), etc., is blue, and that in 
this substance copper seems to take the place of iron. 

The Nitrogen and the Carbon Dioxide of the Blood. — The little 
nitrogen which is found in about equal quantity in venous and 
arterial blood, seems to be simply dissolved. The relations of 
carbonic anhydride are much more complex and obscure. The 


main facts known are that — 1. The quantity of this gas is as 
great in serum as in blood, or, at all events, the quantity in 
serum is very large. 2. The greater part may be extracted by 
'an exhaustion-pump; but a small percentage (2 to 5 volumes 
per cent) does not yield to this method, but is given off when 
an acid is added to the serum. 3. If the entire blood be sub- 
jected to a vacuum, the whole of the CO2 is given off. 

From these facts it has been concluded that the greater part 
of the COs exists in the plasma, associated probably with sodium 
salts, as sodium bicarbonate, but that the corpuscles in some 
way determine its relations of association and disassociation. 
Some think a good deal of this gas is actually united with the 
red corpuscles. 

We may now inquire into the more intimate nature of respi- 
ration in the blood. From the facts we have stated it is obvi- 
ous that respiration can not be wholly explained by the Henry- 
DaltoD law of pressures or any other physical law. It is also 
plain that any explanation which leaves out the principle of 
pressure must be incomplete. 

While there is in oxy-hsemoglobin a certain quantity of oxy- 
gen, which is intra-molecular and incapable of removal by re- 
duction of pressure, there is also a portion which is subject to 
this law, though in a peculiar way; nor is the question of 
temperature to be excluded, for experiment shows that less 
oxygen is taken up by blood at a high than at a low tempera- 

We have learned that, in ordinary respiration, the propor- 
tion of carbonic dioxide and oxygen in different parts of the 
respiratory tract must vary greatly ; the air of necessity being 
much less pure in the alveoli than in the larger bronchi. 

From experiments on blood, venous and arterial, to deter- 
mine the conditions of pressure, temperature, etc., under which 
the injurious gas is got rid of and the necessary one absorbed, 
it has been found that the partial pressure of oxygen in the 
"lungs is sufficient to bring about that surrender of oxygen to 
the blood necessary to keep it all but saturated with this gas 
as it is believed to be ; and that, so far as carbonic anhydride 
is concerned, the same law holds — i. e., the partial pressure in 
the blood is ordinarily greater than in the alveoli. 

By means of an apparatus by which one of the smaller 
bronchi may be occluded for a certain period, and also, allow 
of withdrawal of samples of the air in the occluded portion of 
lung from time to time, to ascertain its composition, attempts 


have been made to determine the pressure relations within an 
alveolus. It is maintained that while the partial pressure of 
the carbonic anhydride rises and of the oxygen sinks, still that 
they remain such as to favor respiration. It is also found that, 
in the asphyxia following occlusion of the trachea, the tension 
of oxygen is always greater, and of carbonic anhydride less, in 
the alveoli than in the blood. On the other hand it is stated 
that oxy-hsemoglobin is found in the blood when every trace of 
oxygen is removed from a chamber in which an asphyxiating 
animal is breathing, so that it is argued that partial pressures 
alone can not explain the facts of respiratioii, and that this 
function is fundamentally a chemical process ; and it is cus- 
tomary to speak of the oxygen of oxy-hsemoglobin as being in 
a state of " loose chemical combination." 

The entire truth seems to lie in neither view, though both 
are partially correct. 

The view expressed by some physiologists, to the effect that 
diffusion explains the whole matter, so far, at least, as carbonic 
anhydride is concerned, and that the epithelial cells of the lung 
have no share in the respiratory process, does not seem to be 
in harmony either with the facts of respiration or with the 
laws of biology in general. Why not say at once that the facts 
of respiration show that, here as in other parts of the economy, 
while physical and chemical laws, as we know them, stand 
related to the vital processes, yet, by reason of being vital 
processes, we can not explain them according to the theories of 
either physics or chemistry ? Surely this very subject shows 
that neither chemistry nor physics is at present adequate to 
explain such processes. It is, of course, of value to know the 
circumstances of tension, temperature, etc., under which respi- 
ration takes place. We, however, maintain that these are con- 
ditions only — essential no doubt, but, though important, that 
they do not make up the process of respiration. But, because 
we do not know the real explanation, let us not exalt a few 
facts or theories of chemistry or physics into a solution of a 
complex problem. Besides, some of the experiments on which 
the conclusions have been based are questionable, inasmuch as 
they seem to induce artificial conditions in the animals oper- 
ated upon ; and we have already insisted on the blood being 
regarded as a living tissue, behaving differently in the body 
and when isolated from it, so that even in so-called blood-gas 
experiments there may be sources of fallacy inherent in the 
nature of the case. 


Foreign Gases and Respiration. — These are divided into : 

1. Indifferent gases, as N, H, CH4, whicli, though not in 
themselves injurious, are entirely useless to the economy. 

2. Poisonous gases, fatal, no matter how abundant the nor- 
mal respiratory food may be. They are divisible into : (a) those 
that kill by displacing oxygen, as NO, CO, HON ; (6) narcotic 
gases, as COs, NjO, producing asphyxia when present in large 
quantities ; (c) reducing gases, as HsS, (N'H4)sS, PHa, AsHa, C-iNl, 
which rob the haemoglobin of its oxygen. 

There are probably a number of poisonous products, some 
■ of them possibly gases, produced by the tissues themselves and 
eliminated normally by the respiratory tract; and these are 
doubtless greatly augmented, either in number or quantity, or 
both, when other excreting organs are disordered. 

Eespiration in the Tissues. 

We first direct attention to certain striking facts : 
1. An isolated (frog's) muscle will continue to contract for 
a considerable period and to exhale carbon dioxide in the total 
absence of oxygen, as in an atmosphere of hydrogen ; though, 
of course, there is a limit to this, aiid a muscle to which either 
no blood flows, or only venous blood, soon shows signs of 
fatigue. 2. In a frog, in which physiological saline solution 
has been substituted for blood, the metabolism will continue, 
carbonic anhydride being exhaled as usual. 3. Substances, 
which are readily oxidized, when introduced into the blood of 
a living animal or into that blood when withdrawn undergo 
but little oxidative change. 4. An entire frog will respire car- 
bonic dioxide for hours in an atmosphere of nitrogen. 

Such facts as these seem to teach certain lessons clearly. It 
is evident, first of all, that the oxidative processes that give rise 
to carbon dioxide occur chiefly in the tissues and not in the 
blood ; that in the case of muscle the oxygen that is used is first 
laid by, banked as it were against a time of need, in the form of 
intra-molecular oxygen, which is again set free in the form of 
carbon dioxide, but by what series of changes we are quite un- 
able to say. Though our knowledge of the respiratory processes 
of muscle is greater than for any other tissue, there seems to 
be no reason to believe that they are essentially different else- 
where. The advantages of this banking of oxygen are, of course, 
obvious ; were it otherwise, the life of every cell must be at the 
mercy of the slightest interruption of the flow of blood, the 


entrance of air, etc. Even as it is, the need of a constant supply 
of oxygen in warm-blooded animals is much greater than in 
cold-blooded creatures, which can long endure almost entire 
cessation of both respiration and circulation, owing to the com- 
paratively slow rate of speed of the vital machinery. 

If one were to rely on mere appearances he might suppose 
that in the more active condition of certain organs there was 
less chemical interchange (respiration) between the blood and 
the tissues than in the resting stage, or, properly speaking, 
more tranquil stage, for it must be borne in mind that a living 
cell is never wholly at rest; its molecular changes are cease- 
less. It happens, e. g., that when certain glands (salivary) are 
secreting actively, the blood flowing from them is less venous 
in appearance than when not functionally active. This is not 
because less oxygen is used or less abstracted from the blood, 
but because of the greatly increased speed of the blood-flow, so 
that the total supply to draw from is so much larger that, 
though more oxygen is actually used, it is not so much missed, 
nor do the greater additions of carbon dioxide so rapidly pol- 
lute this rapid stream. 

It is thus seen that throughout the animal kingdom respira- 
tion is fundamentally the same process. It is in every case 
finally a consumption of oxygen and production of carbonic 
anhydride by the individual cell, whether that be an Amoeba 
or an element of man's brain. These are, however, but the 
beginning and end of a very complicated biological history of 
by far the greater part of which nothing is yet known ; and it 
must be admitted that diffusion or any physical explanation 
carries us but a little way on toward the understanding of it. 

The N'brvous System in Relation to Respiration. 

We have considered the muscular movements by which the 
air is' made to enter and leave the lungs in consequence of 
changes in the diameters of the air -inclosing case, the thorax. 
It remains to examine into the means by which these muscles 
were set into harmonious action so as to accomplish the pur- 
pose. The nerves supplying the muscles of respiration are de- 
rived from the spinal cord, so that they must be under the 
dominion of central nerve-cells situated either in the cord or 
the brain. Is the influence that proceeds outward generated 
within the cells independently of any afferent impulses, or is it 
dependent on such causes ? Let us appeal to facts. 


1. If the phrenics, an intercostal nerve, etc., be cut, there is 
a corresponding paralysis of the muscle supplied. 2. If the 
spinal cord be divided below the medulla oblongata, there is a 
cessation of all respiratory movements except those of the 
larynx and face, which also disappear if the facial and recur- 
rent laryngeal nerves be divided. 3. So long as the medulla 
remains,. respiration may continue ; but if even a small part of 
this region, situated below the vaso-motor center between this 
and the calamus scriptorius (respiratory center, noRud vital), 
be injured, death ensues rapidly. Plainly, then, there are cen- 
tral cells which originate the impulses that energize the mus- 

It remains to inquire still whether they are independent 
(automatic) centers, or are influenced by impulses reaching 
them from without. Is the government absolute, or subject to 
the will of the multitudinous cells of the organic common- 
wealth ? 

Again let us appeal to facts: 1. If one vagus nerve be cut, 
a change is observable in the respiratory rhythm, which is 
much more pronounced if both nerves be divided. Respiration 
becomes slower, and the pause between inspiration and expira- 
tion greatly lengthened, though the gaseous interchange re- 
mains much as before. 2. If one suddenly step into a cold 
bath, he naturally draws a long breath. Again, the respiration 
is very greatly altered in consequence of emotional changes; 
indeed, there is probably no rhythm in the body more subject 
to frequent obvious alteration than that of respiration. 3, 
Stimulation of the central end of such a nerve as the sciatic 
causes marked change in the rhythm of breathing. 4. Stimu- 
lation of the central end of the vagus usually quickens res- 
piration, while stimulation of the central end of the siiperior 
laryngeal has the opposite effect. If the current be strong, 
respiration may be arrested in each instance, though in a differ- 
ent manner. In the case of vagus stimulation the result is 
inspiratory spasm, and of the superior laryngeal expiratory 

These and a host of additional facts, experimental and other, 
show that the central impulses are modified by afferent im- 
pulses reaching the center through appropriate nerves. More- 
over, drugs seem to act directly on the center through the 

The vagus is without doubt the afferent respiratory nerve, 
though how it is affected, whether by the mechanical movement 



of the lungs merely, by the condition of the blood as regards its 
contained gases, or, as seems most likely, by a combination of 
circumstances into which these enter and are probably the 

Brain above medulla from which 
impulses modifying respiralton 
may proceed. 

'acial muselea. 

Respiratory centre 
in the medulla. 

Cutaneous surface from which 
afferent impulses proceed di- 
rectly to brain. 

Thoradc resp. muscles. 

Spinal cord. 

f—Bespiratory tract. 

Diaphragm with 
phrenic nerve. 

Cutaneous sur- 
face from which 
impulses reach res- 
piratory centre by 
spinal cord. 

Fio. dl8.— Diagram intended to illustrate nervous meclianisin of respiration. Arrows indicate 
course of impulses. 

principal, is not demonstrably clear. When others function as 
afferent nerves, capable of modifying the action of the respira- 


tory center, they are probably influenced by the respiratory 
condition of the blood, though not necessarily exclusively. 

But when all the principal afferent impulses are cut off by 
division of the nerves reaching the respiratory center directly 
or indirectly, respiration will still continue, provided the motor 
nerves and the medulla remain intact. 

The center, then, is not, mainly at least, a reflex but an auto- 
matic one, though its action is modified by afferent impulses 
reaching it from every quarter. Since respiration continues 
when the medulla is divided in the middle line, yet is modified 
unilaterally when one vagus is divided, it is inferred that the 
respiratory center is double, that each half usually works in 
harmony with the other, but that each can act independently. 
Though it seems clear enough that the respiratory center is 
automatic, and that its action is modified according to the con- 
dition of the organism generally, as communicated to it by the 
various afferent nerves and the blood itself, yet the exact man- 
ner of its action — why inspiration follows up expiration — has 
not been clearly explained. Some assume that during expira- 
tion inspiratory impulses are gathering head and finally check 
expiration by originating inspiration, while these are opposed 
by another process which at length gives rise to enough resist- 
ance to check inspiration, and originate expiration; and this 
theory becomes more complete if an expiratory as well as in- 
spiratory center be assumed. 

We have hitherto spoken only of a single respiratory cen- 
ter in the medulla, but certain experimental facts throw addi- 
tional light on the subject. 

In young mammals — e. g., kittens — it is found that, in the 
absence of the medulla, respiratory movements may be induced 
by stimulating (pinching) the surface, especially if the action 
of the spinal cord be augmented by the administration of 
strychnia. From this it has been inferred that there are respir- 
atory centers in the spinal cord, subordinate to the main cen- 
ter in the medulla. Considering the imperfect nature of the 
respiratory act as thus induced, and the circumstances of the 
case, the conclusion has the appearance of being a little strained. 
But quite recently it has been shown that in the adult dog 
when the cord is severed below the medulla, and artificial res- 
piration maintained for some time, on ceasing this, breathing 
begins spontaneously and continues for a considerable period ; 
and the expiratory phase of respiration in this case is the most 
marked. It has been argued from this experiment that there 


are both, inspiratory and expiratory centers in tlie spinal cord. 
But, as we have pointed out, on more than one occasion, we 
must always he on our guard in interpreting the behavior of 
one part when another is out of gear. There is so much, latent 
resource, so great a power to resume functions normally laid 
aside, if not wholly in great part, that we should hesitate be- 
fore inferring that the spinal cord usually takps a prominent 
share in originating the impulses which govern respiration. 
Notwithstanding the suggestiveness of such experiments, we 
do not think they make the medulla appear in a less important 
light as the part of the nervous system dominant in respira- 
tion ; though there may be nervous machinery in the cord usu- 
ally in feeble action, susceptible of assuming a more exalted 
functional role when occasion urgently demands and when en- 
couraged, so to speak, to do so, as in the experiments referred 
to above ; indeed such, upon our own theory of physiological 
reversion, would naturally be the case. We must, however, 
draw the line between what is and what may be in function. 

The Influence of the Condition of the Blood in Bespiration. — If 
for any reason the tissues are not receiving a due supply of 
oxygen, they manifest their disapproval, to speak figuratively, 
by reports to the responsible center in the medulla, and if the 
medulla is a sharer in the lack, as it naturally would be, it takes 
action independently. One of the most obvious instances in 
which there is oxygen starvation is when there is hindrance to 
the entrance of air, owing to obstruction in the respiratory 

At first the breathing is merely accelerated, with perhaps 
some increase in the depth of the inspirations (hyperpnoRa), a 
stage which is soon succeeded by labored breathing {dyspno&a), 
which, after the medulla has called all the muscles usually em- 
ployed in respiration into violent action, passes into convul- 
sions, in which every muscle may take part. 

In other words, the respiratory impulses not only pass along 
their usual paths as energetically as possible, but radiate into 
unusual ones and pass by nerves not commonly thus set into 
functional activity. 

It would be more correct, perhaps to assume that the vari- 
ous parts of the nervous system are so linked together that ex- 
cessive activity of one set of connections acts like a stimulus to 
rouse another set into action, the order in which this happens 
depending on the law of habit — habit personal and especially 
ancestral. An opposite condition to that described, known as 


apnoRa, may be induced by pumping air into an animal's chest 
very rapidly by a bellows ; or in one's self by a succession of 
rapid, deep respirations. 

After ceasing, the breathing may be entirely interrupted 
for a brief interval, then commence very quietly, gradually in- 
creasing to the normal. 

Apncea has been interpreted in two ways. Some think that 
it is due to fatigue of the muscles of respiration or the respira- 
tory center; others that the blood has under these circum- 
stances an excess of oxygen, which so influences the respiratoiy 
center that it is quieted (inhibited) for a time. 

The latter view is that usually adopted ; but, considering that 
apncea results from the sobbing of children following a pro- 
longed fit of crying, also in Cheyne-Stokes and other abnormal 
forms of breathing, and that the blood is normally almost satu- 
rated with oxygen, it will be agreed that there is a good deal 
to be said for the first view, especially that part of it which 
represents the cessation of breathing as owing to excessive 
activity and exhaustion of the respiratory center. "We find 
such a calm in asphyxia after the convulsive storm. 

Is it, then, the excessive accumulation of carbon dioxide or 
the deficiency of oxygen that induces dyspnoea ? Considering 
that the former gas acts as a narcotic, and does not induce con- 
vulsions, even when it constitutes a large percentage of the 
atmosphere breathed, and that the need of oxygen for the tis- 
sues is constant, it certainly seems most reasonable to conclude 
that the phenomena of dyspnoea are owing to the lack of oxy- 
gen chiefly, at least ; though the presence of an excess of car- 
bonic anhydride may take some share in arousing that vigorous 
effort on the part of the nervous system, to restore the func- 
tional equilibrium, so evident under the circumstances. 

The Cheyne-Stokes Bespiration (Phenomenon). — There is a form 
of breathing occurring under a variety of abnormal circum- 
stances, in which the respirations gradually reach a maximum 
(dyspnoea), and then as gradually decline to absolute cessation 
(apnoea). The pause may last a surprising length of time (one 
half to three quarters of a minute), when this form of breathing 
again repeats itself. It has been compared to the periodic 
grouping of heart-beats (Luciani groups), occurring when the 
organ is suffering. There is abundant cause usually for ex- 
haustion of the center, on account of disordered blood or an 
insufl&cient supply to the brain. This phenomenon and apnoea 
bring out clearly the rhythmic character of those processes. 


like respiration, wliicli in tlie nature of the case must be in 
the higher groups of vertebrates ceaseless, and it is not surpris- 
ing that, like a lame dog, which prefers progression by three 
legs to none at all, the ever-active center will keep up its rhythm 
as long as it can — perfectly, if possible, and, if not perfectly, as 
well as it can. We mean to imply that its action must be 
rhythmic, or cease entirely. 

The Effects of Variations in the Atmospheric 

These depend in great part upon the suddenness with which 
the change is made. When an individual ascends a high 
mountain or rises in a balloon, parts in contact with the air 
become reddened and swollen, owing to the distention of the 
small vessels, which may result in haemorrhages. There is dif- 
ficulty in breathing, the respirations become more rapid, as also 
the pulse. If the lowering of pressure amounts to from one 
third to one half, the quantity of oxygen in the blood is dimin- 
ished, and the carbon dioxide imperfectly excreted. Owing to 
the excess of blood in the superficial parts, the internal organs 
become anaemic, and there is consequently diminished secretion 
of urine and a variety of other disturbances, with general weak- 
ness. The blood-pressure is also altered. 

Sudden diminution of pressure gives rise to a liberation of 
gas — chiefly nitrogen — within the blood-vessels, which causes 
death by blocking the circulation in the small vessels (hence 
also the danger from section of a large vein in surgical opera- 
tions about the neck, the air being liable to be sucked in, owing 
to the negative pressure). 

Increase in the atmospheric pressure when not very great 
gives rise to symptoms akin to those of narcotic poisoning; 
but when the increase amounts to twenty atmospheres, animals 
die, as if asphyxiated, with convulsions. Neither the assump- 
tion of oxygen nor the separation of carbon dioxide takes place 
to the usual extent ; and it is interesting to note that micro- 
organisms are killed under similar circumstances. 

With considerable diminution of pressure, though not suf- 
ficient to lead to a fatal result, symptoms the opposite of those 
described above occur. Thus, there is paleness of the surface, 
respiration is easy, the capacity of the lungs is increased, owing, 
it is thought, to the greater descent of the diaphragm, in con- 
sequence of the compression of the gases of the intestines. 



Urine is secreted in excess, there is more muscular energy, and 
the metabolism of the body generally is accelerated. Air under 
altered pressure has been employed as a therapeutic agent, but 
a little reflection will make it clear that it is a remedy to be 
used with the greatest care, especially when there is disease of 
the heart, blood-vessels, etc. 

The Influence of Eespiration on the Cikculation. 

An examination of tracings of the intra-thoracic and blood- 
pressure, taken simultaneously, shows (1) that during inspira- 
tion the blood-pressure rises and the intra-thoracic pressure 
falls ; (2) that during expiration the reverse is true ; and (3) that 
the heart-beat is slowed, and has a decided effect on the form 
of the pulse. But it also appears that the period of highest 
blood-pressure is just after expiration has begun. 

Fig. 319- Tracings of blood-pressure and intrathoracic pressiire (after Foster), a, blood- 
pressure tracing showing irregularities due to respiration and pulse : 6, curve of intra- 
thoracic pressure ; t", beginning of inspiration ; e, of expiration. Intrathoracic pressiu'e 
is seen to rise rapidly after inspiration ceases, and then slowly sinks as the expiratory 
blast continues, to become a rapid fall when inspiration begins. 

We must now attempt to explain how these changes are 
brought about. By intra-thoracic pressure is meant the press- 
ure the lungs exert on the costal pleura or any organ within 
the chest, which must differ from intra-pulmonary pressure 
and the pressure of the atmosphere, because of the resistance 
of the lungs by virtue of their own elasticity. 

It has been noted that even in death the lungs remain par- 
tially distended ; and that when the thorax is opened the pul- 
monary collapse which follows demonstrates that their elas- 
ticity amounts to about five millimetres of mercury, which 
must, of course, represent but a small portion of that elasticity 
which may be brought into play when these organs are greatly 
distended, so that they never press on the costal walls, heart. 


etc., witli a pressure equal to that of the atmosphere. It follows 
that the deeper the inspiration the greater the difference be- 
tween the intra-thoracic and the atmospheric pressure. Even 
in expiration, except when forced, the intra-thoracic pressure 
remains less, for the same reason. 

These conditions must have an influence on the heart and 
blood-vessels. Bearing in mind that the pressure without is 
practically constant and always greater than that within the 
thorax, the conditions are favorable to the flow of blood toward 
the heart. As in inspiration, the pressure on the great veins 
and the heart is diminished, and, as these organs are not rigid, 
they tend to expand within the thorax, thus favoring an on- 
ward flow. But the opposite effect would follow as regards the 
large arteries. Their expansion must tend to withdraw blood. 
During expiration the conditions are reversed. The effects on 
the great veins can be observed by laying them bare in the 
neck of an animal, when it may be seen that during inspiration 
they become partially collapsed, and refilled during expiration. 
In consequence of the marked thickness of the coats of the 
great arteries, the effect of changes in intra-thoracic pressure 
must be slight. The comparatively thin-walled auricles act 
somewhat as the veins, and it is likely that the increase of 
pressure during expiration must favor, so far as it goes, the 
cardiac systole. 

More blood, then, entering the right side of the heart dur- 
ing inspiration, more will be thrown into the systemic circula- 
tion, unless it be retained in the lungs, and, unless the effect be 
counteracted, the arterial pressure will rise, and, as all the con- 
ditions are reversed during expiration, we look for and find 
exactly opposite results. The lungs themselves, however, must 
be taken into the account. During inspiration room is pro- 
vided for an increased quantity of blood, the resistance to its 
flow is lessened, hence more blood reaches the left side of the 
heart. The 'irmrnediate effect would be, notwithstanding, some 
diminution in the quantity flowing to the left heart, in conse- 
quence of the sudden widening of the pulmonary vessels, the 
reverse of which would follow during expiration; hence the 
period of highest intra-thoracic pressure is after the onset of 
the expiratory act. During inspiration the descent of the dia- 
phragm compressing the abdominal organs is thought to force 
on blood from the abdominal veins into the thoracic vena cava. 

That the respiratory movements do exert in some way a 
pronounced effect on the circulation the student may demon- 



strate to himself in the folio-wing ways : 1. After a full inspira- 
tion, close the glottis and attempt to expire forcibly, keeping 
the fingers on the radial artery. It may be noticed that the 
pulse is modified or possibly for a moment disappears. 2. Be- 
verse the experiment by trying to inspire forcibly with closed 
glottis after a strong expiration, when the pulse will again be 
found to vary. In the first instance, the heart is comparative- 
ly empty and hampered in its action, intra-thoracic pressure 
being so great as to prevent the entrance of venous blood by 
compression of the heart and veins, while that already within 
the organ and returning to it from the lungs soon passes on 
into the general system, hence the pulseless condition. The 
explanation is to be reversed for the second case. The heart's 
beat is modified, probably reflexly, through the cardio-inhibitory 
center, for the changes in the pulse-rate do not occur when the 
vagi nerves are cut, at least not to nearly the same extent. 

Comparative. — It may be stated that the cardiac phenomena 
referred to in this section are much more marked in some ani- 
mals than in others. Very little change may be observed in 
the pulse-rate in man, while in the dog it is so decided that one 
observing it for the first time might suppose that such pro- 
nounced irregularity of the heart was the result of disease ; 
though even in this animal there are variations in this respect 
with the breed, age, etc. 

We must now direct attention to certain facts which have 
been very differently interpreted. 

During artificial respiration, when air is pumped into the 
chest by a bellows, it follows, of course, that all the usual press- 
ure conditions are reversed — e. g., the inspiratory pressure is 
greater than the expiratory. 

If artificial respiration, in an animal under experiment, be 
stopped, it may be noticed that there is at first a steady rise of 
blood-pressure ; but presently certain undulations in the respir- 
atory tracings may be observed, known as Traube-Hering 
curves ; and these will appear even when the vagi nerves are 
cut, and disappear only with the fall of blood-pressure that 
ensues with the exhaustion of the animal. 

If the spinal cord has been divided, the tracings may still 
be obtained, though the effect is not so marked. These are the 
phenomena, but there is much divergence of opinion as to their 
cause. Some maintain that mechanical effects suffice to explain 
them, though the majority are not of this opinion, but believe 
them due to rhythmical variations in the caliber of the arteri- 



oles affected through vaso-motor nerves in obedience to the 
medullary center which operates by their agency; and that 

Fio. 320.— Tracings of blood-pressure in rabbit to show Traube-Hering curves (after Foster). 
The widest undulations indicate Traube Bering eiurves ; those next in size, effects of 
respiration ; and the smallest, of the pulse. 

when this center is disabled its subordinates in the spinal cord 
take upon them the task. It has also been suggested that there 
may be a local vaso-motor mechanism acted upon by the ve- 
nous blood or that the muscle-cells themselves may be influ- 
enced by the unnatural condition of the blood in asphyxia. 

These curves, however, also appear during respiration that 
deviates but little from the normal. 

It is to be borne in mind that the tracings on which we 
have based our reasoning do not represent what takes place in 
every mode of breathing. The subject is one of great com- 
plexity. Doubtless mechanical explanations go a long way 
here, but they are so mixed up with factors that play a part 
more or less prominent, though difficult to isolate in individual 
instances, and in no wise to be explained as other than vital 
effects, that one must exercise the usual caution ; the more so 
as it is found upon actual experiment that the outcome, as 
regards blood-pressure, is not always quite what would have 
been expected, reasoning from the principles of physics alone. 

That there are rhythms within rhythms in the vascular and 
respiratory system, rendering the subject complex beyond the 
power of experiments fully to unravel, is a conviction that we 
think will deepen in the minds of physiologists. 


The Respiration and Circulation in Asphyxia. — A most instruct- 
ive experiment may he arranged thus : 

Let an ansestlietized rabbit, cat, or sucb-like animal, have 
the carotid of one side connected with a glass tube as before 
described (page 339), by which the blood-pressure and its 
changes may be indicated, and, when the normal respiratory 
acts have been carefully observed, proceed to notice the effects 
on the blood-pressure, etc., of pumping air into the chest by a 
bellows, of hindering the ingress of air to a moderate degree, 
and of struggling. With a small animal it will be difiicult to 
observe the respiratory effects on the blood-pressure by simply 
watching the oscillations of the fluid in the glass tube, but this 
is readily enough made out if more elaborate arrangements be 
made, so that a graphic tracing may be obtained. 

But the main events of asphyxia may be well (perhaps best) 
studied in this manner : 

Let the trachea be occluded (ligatured). At once the blood- 
pressure will be seen to rise and remain elevated for some time, 
then gradually fall to zero. These changes are contemporane- 
ous with a series of remarkable manifestations of disturbance 
in the respiratory system as it at first appears, but in reality 
due to wide-spread and profound nutritive disturbance. So far 
as the breathing is concerned, it may be seen to become more 
rapid, deeper, and labored, in which the expiratory phase be- 
comes more than proportionably marked (dyspnoea) ; this is fol- 
lowed by the gradual action of other muscles than those usually 
employed in respiration, until the whole body passes into a ter- 
rible convulsion — a muscle-storm in consequence of a nerve- 
storm. When this has lasted a variable time, but usually 
about one minute, there follows a period of exhaustion, during 
which the subject of the experiment is in a motionless condi- 
tion, interrupted by an occasional respiration, in which inspi- 
ration is more pronounced than expiration ; and, finally, the 
animal quietly stretches every limb, the sphincters are relaxed, 
there may be a discharge of urine or faeces from peristaltic 
movements of the bladder or intestines, and death ends a strik- 
ing scene. These events may be classified in three stages, 
though the first and second especially merge into one another : 
1. Stage of dyspnoea. 2. Stage of convulsions. 3. Stage of 

It is during the first two stages that the blood-pressure rises, 
and during the third that it sinks, due in the first instance 
chiefly to excessive activity of the vaso-motor center, and. in 


the second to its exhaustion and the weakening of the heart- 

These violent movements are owing, we repeat, to the action 
of blood deficient in oxygen on the respiratory center (or cen- 
ters), leading to inordinate action followed by exhaustion. 

The duration of the stages of asphyxia varies with the ani- 
mal, but rarely exceeds five minutes. In this connection it may 
be noted that newly-born animals (kittens, puppies) bear im- 
mersion in water for as much as from thirty to fifty minutes, 
while an adult dog dies within four or five minutes. This is 
to be explained by the feeble metabolism of new-born mam- 
mals, which so slowly uses up the vital air (oxygen). 

If the chest of an animal be opened, though the respiratory 
muscles contract as usual there is, of course, no ventilation of 
the lungs, which lie collapsed in the chest ; and the animal dies 
about as quickly as if its trachea were occluded. It passes 
through all the phases of asphyxia as in the former case ; but 
additional information may be gained. The heart is seen to 
beat at first more quickly and forcibly, later vigorously though 
slower, and finally both feebly and irregularly, till the ventri- 
cles, then the left auricle, and finally the right auricle cease to 
beat at all or only at long intervals. The terminations of the 
great veins (representing the sinus venosus) beat last of all. 

At death the heart and great veins are much distended 
with blood, the arteries comparatively empty. Even after 
rigor mortis has set in, the right heart is still much engorged. 

These phenomena are the result of the operation of several 
causes. The increasingly venous blood at first stimulates the 
heart probably directly, in part at least, but later has the con- 
trary effect. The nutrition of the organ suffers from the de- 
graded blood, from which it must needs derive its supplies. 
The cardio-inhibitory center probably has a large share in the 
slowing of the heart, if not also in quickening it. Whether 
the accelerator fibers of the vagus or sympathetic play any 
part is uncertain. The increase of peripheral resistance caused 
by .the action of the vaso-motor center makes it more difficult 
for the heart to empty its left side and thus receive the venous 
blood as it pours on. At the same time the deep inspirations 
(when the chest is unopened) favor the onflow of venous blood ; 
and in any case the whole venous system, including the right 
heart, tends to become engorged from these several causes act- 
ing together. The heart gives up the struggle, unable to main- 
tain it, but not so long as it can beat in any part. 


The share which the elasticity of the arteries takes in 
forcing on the blood when the heart ceases, and the contraction 
of the muscular coat of these vessels, especially the smaller, 
must not be left out of the account in explaining the phenom- 
ena of asphyxia and the post-mortem appearances. 

Pathological. — The importance of being practically as well as 
theoretically acquainted with the facts of asphyxia is very great. 

The appearance of the heart and venous system gives une- 
quivocal evidence as to the mode of death in any case of as- 
phyxia ; and the contrast between the heart of an animal bled 
to death, or that has died of a lingering disease, and one 
drowned, hanged, or otherwise asphyxiated, is extreme. 

We strongly recommend the student to asphyxiate some 
small mammal placed under the influence of an anaesthetic, 
and to note the phenomena, preferably with the chest opened ; 
and to follow up these observations by others after the onset 
of rigor mortis. 

Peculiar Respiratory Movements. 

Though at first sight these seem so different, and are so as 
regards acts of expression, yet from the respiratory point of 
view they resemble each other closely; they are all reflex, 
and, of course, involuntary. Many of them have a common 
purpose, either the better to ventilate the lungs, to clear them 
of foreign bodies, or to prevent their ingress. 

Coughing, in which such a purpose is evident, is made up of 
several expiratory efforts preceded by an inspiratory act. The 
afferent nerve is tisually the vagus or laryngeal, but may be 
one or more of several others. 

The glottis presents characteristic appearances, being closed 
and then opened suddenly, the mouth being kept open. 

Coughing is often induced in attempting to examine the ear 
with instruments. (Reflex act.) 

Laughing is very similar to the last, so far as the behavior 
of the glottis is concerned, though it usually acts more rapidly, 
of course. Several expirations follow a deep inspiration. 

Crying is essentially the same as laughing, but the facial 
expression is different, and the lachrymal gland functions exces- 
sively, though with some persons this occurs during laughter 

Sobbing is made up of a series of inspirations, in which the 
glottis is partially closed, followed by a deep expiration. 


Yawning involves a deep-drawn, slow inspiration, followed 
by a more sudden expiration, with a well-known depression of 
the lower jaw and usually stretching movements. 

Sighing is much like the preceding, though the mouth is not 
opened widely if at all, nor do the stretching movements com- 
monly occur. 

Hiccough is produced by a sudden inspiratory effort, though 
fruitless, inasmuch as the glottis is suddenly closed. It is 
spoken of as spasm of the diaphragm, and when long continued 
is very exhaustive. 

Sneezing is the result of a powerful and sudden expiratory 
act following a deep inspiration, the mouth being usually closed 
by the anterior pillars of the fauces against the outgoing cur- 
rent of air, which then makes its exit through the nose, while 
the glottis is forcibly opened after sudden closure. It will be 
noticed that in most of thege acts the glottis is momentarily 
closed, which is never the case in mammals during quiet res- 

This temporary occlusion of the respiratory passages per- 
mits of a higher intrapulmonary pressure, which is very effect- 
ive in clearing the passages of excess of mucus, etc., when the 
glottis is suddenly opened. Though the acts described are all 
involuntary, they may most of them be imitated and thus 
studied deliberately by the student. It will also appear, con- 
sidering the many ways in which some if not all of them may 
be brought about, that if the medullary center is responsible 
for the initiation of them, it must be accessible by numberless 

Comparative. — Few of the lower animals cough with the same 
facility as man, while laughing is all but unknown, crying and 
sobbing rare, though the whining of dogs is allied to the cry- 
ing of human beings. 

Sneezing seems to be voluntary in some animals, as squir- 
rels, when engaged in toilet operations, etc. 

Barking is voluntary, and in mechanism resembles cough- 
ing, the vocal cords being, however, more definitely employed, 
as also in growling. 

Bawling, neighing, braying, etc., are made up of long,expira- 
tory acts, preceded by one or more inspirations. The vocal 
cords are also rendered tense. 

408 animal physiology. 

Special Considerations. 

Pathological and Clinical. — The minLber of diseases that lessen 
the amount of availahle pulmonary tissue, or hamper the move- 
ments of the chest, are many, and only the briefest reference 
can be made to a few of them. 

Inflammation of the lungs .may render a greater or less por- 
tion of one or both lungs solid ; inflammation of the pleii/ra 
(pleuritis, pleurisy) by the dryness, pain, etc., may restrict the 
thoracic movements; phthisis may solidify or excavate the lungs, 
or by pleuritic inflammation glue the costal and pulmonary 
pleural surfaces together ; bronchitis clog the tubes and other 
air-passages with altered secretions ; emphysema (distention of 
air-cells) may destroy elasticity of parts of the lung ; pneumo- 
thorax from rupture of the lung-tissue and consequent accumu- 
lation of gases in the pleural cavity, or pleurisy with effusion, 
render one lung all but useless from pressure. In all such 
cases Nature attempts to make up what is lost in amplitude by 
increase in rapidity of the respiratory movements. It is inter- 
esting to note too how the other lung, in diseased conditions, if 
it remain unaffected, enlarges to compensate for the loss on the 
opposite side. When the muscles are weak, especially if there 
be hindrance to the entrance of air while the thoracic move- 
ments are marked, there may be bulging inward of the inter- 
costal spaces. 

Normally, this would also occur, as the intra-thoracic press- 
ure is less than the atmospheric, were it not for the fact that 
the intercostal muscles when contracting have a certain resist- 
ing power. 

The imperfect respiration of the moribund, permitting the 
accumulation of carbonic anhydride with its soporific effects, 
smooths the descent into the valley of the shadow of death ; so 
that there may be to the uninitiated the appearance of a suffer- 
ing which does not exist, consciousness itself being either 
wholly or partially absent. The dyspnoea of anaemic persons, 
whether from sudden loss of blood or from imperfect renewal 
of the haemoglobin, shows that this substance has a respiratory 
function ; while in forms of cardiac disease with regurgitation, 
etc., the blood may be imperfectly oxidized, giving rise to la- 
bored respiration. 

Personal Observation. — As hinted from time to time during 
the treatment of this subject, there is a large number of facts 
the student may verify for himself. 


A simple way of proving that CO2 is exhaled is to breathe 
(blow) into a vessel containing some clear solution of quick- 
lime (CaO), the turbidity showing that an insoluble salt of lime 
(CaCOa) has been formed by the addition of this gas. 

The functions of most of the respiratory muscles, the phe- 
nomena of dyspncea, apnoea (by a series of long breaths), partial 
asphyxia by holding the breath, and many other experiments, 
simple but convincing, will occur to the student who is willing 
to learn in this way. 

The observation of respiration in a dreaming animal (dog) 
will show how mental occurrences affect the respiratory center 
in the absence of all the usual outward influences. The respira- 
tion of the domestic animals, of the frog, turtle, snake, and fish 
are easily watched if these cold-blooded animals be placed for 
observation beneath a glass vessel. Their study will teach how 
manifold are the ways by which the one end is attained. Com- 
pare the tracings of Fig. 313. 

Evolution. — A study of embryology shows that the respira- 
tory and circulatory systems develop together ; that the vascu- 
lar system functions largely as a respiratory system also in cer- 
tain stages, an,d remains such, from a physiological point of 
view, throughout embryonic life. 

The changes that take place in the vascular system — the 
heart, especially — of the mammal when the lungs have become 
functionally active at birth, show how one set of organs modi- 
fies the other. 

When one considers, in addition to these facts, that the 
digestive as well as the vascular and respiratory organs are 
represented in one group of structures in a jelly-fish, and that 
the lungs of the mammal are derived from the same mesoblast 
as gives rise to the digestive and circulatory organs, many of 
the relations of these systems in the highest groups of animals 
become intelligible ; but unless there be descent with modifica- 
tion, these facts, clear enough from an evolutionary standpoint, 
are isolated and out of joint, bound together by no common 
principle that satisfies a philosophical biology. 

It has been found that in hunting-dogs and wild rabbits the 
vagus is more efficient than in other races of dogs and in rab- 
bits kept in confinement ; and possibly this may in part account 
for the greater speed and especially the endurance of the 
former. The very conformation of some animals, as the grey- 
hound, with his deep chest and capacious lungs, indicates an 
unusual respiratory capacity. 


The law of habit is well illustrated in the case of divers, who 
can bear deprivation of air longer than those ■unaccustomed to 
such submersion in water. Greater toleration on the part of 
the respiratory center has probably much to do with the case, 
though doubtless many other departures from the normal occur, 
either independently or correlated to the changes in the respira- 
tory center. 

Summary of the Physiology of Respiration.— The purpose of 
repiration in all animals is to furnish oxygen for the tissues 
and remove the carbonic anhydride they produce, which in all 
vertebrates is accomplished by the exposure of the blood in 
capillaries to the atmospheric air, either free or dissolved in 
water. A membrane lined with cells always intervenes between 
the capillaries and the air. 

The air may be pumped in and out, or sucked in and forced 

Bespiration in the Mammal. — The air enters the lungs, owing 
to the enlargement of the chest in three directions by the action 
of certain muscles. It leaves the lungs because of their own 
elastic recoil and that of the chest- wall chiefly. Inspiration is 
active, expiration chiefly passive. 

The diaphragm is the principal muscle of respiration. In 
some animals there is a well-marked facial and laryngeal as 
well as thoracic respiration. Respiration is rhythmical, con- 
sisting of inspiration, succeeded without appreciable pause by 
expiration, the latter being in health of only slightly longer 
duration. There is also a deiinite relation between the number 
of respirations and of heart-beats. According as respiration is 
normal, hurried, labored, or interrupted, we describe it as 
eupncea, hyperpnoRa, dyspnma, and apncea. The intra-thoracic 
pressure is never equal to the atmospheric — i. e., it is always 
negative — except in forced expiration ; and the lungs are never 
collapsed so long as the chest is unopened. The expired air 
differs from that inspired in being of the temperature of the 
body, saturated with moisture, and containing about 4 to 5 
per cent less oxygen and 4 per cent more carbonic anhydride, 
besides certain indifferently known bodies, the result of tissue 
metabolism, excreted by the lungs. 

The quantity of air actually moved by a respiratory act, as 
compared with the total capacity of the respiratory organs, is 
small ; hence a great part must be played by diffusion. The 
portion of air that can not be removed from the lungs by any 
respiratory effort is relatively large. 


It is customary to distinguisli tidal, complementary, supple- 
mentary, and residual air. 

Tlie vital capacity is estimated by tlie quantity of air the 
respiratory organs can move, and is very variable. 

The blood is the respiratory tissue, through the mediation 
of its red cells, by the haemoglobin they contain. This sub- 
stance is a ferruginous proteid, capable of crystallization, and 
assuming under chemical treatment many modifications. When 
it contains all the oxygen it can retain, it is said to be saturated, 
and is called oxy -haemoglobin, in which form it exists (with 
some reduced haemoglobin) in arterial lalood, and to a lesser 
extent in venous blood, which differs from arterial in the rela- 
tive proportions of haemoglobin (reduced) it contains, as viewed 
from the respiratory standpoint. 

Oxy -haemoglobin does not assume or part with its oxygen, 
according to the Henry-Dalton law of pressures, nor is this gas 
in a state of ordinary chemical combination. It is found that 
the oxygen tension of the blood is lower and that of carbonic 
anhydride higher than in the air of the alveoli of the lungs, 
while the same may be said of the tissues and the blood re- 
spectively. This has been, however, recently again denied. 

Respiration is a vital process, though certain physical con- 
ditions (temperature and pressure) must be rigidly maintained 
in order that the gaseous interchanges shall take place. Res- 
piration is always fundamentally bound up with the metabo- 
lism of the tissues themselves. All animal cells, whether they 
exist as unicellular animals (Amoeba) or as the components of 
complex organs, use up oxygen and produce carbonic dioxide. 
Respiratory organs, usually so called, and the respiratory tissue 
par excellence (the blood) are only supplementary mechanisms 
to facilitate tissue respiration. Carbonic anhydride exists in 
blood probably in combination with sodium salts, though the 
whole matter is very obscure. 

Respiration, like all the other functions of the body, is con- 
trolled by the central nervous system through nerves. The 
medulla oblongata is chiefly concerned, and especially one 
small part of it known as the respiratory center. It is possi- 
ble, even probable, that there are subordinate centers in the 
cord, which, under peculiar circumstances, assume importance ; 
but how far they act in concert with the medullary center, or 
whether they act at all when normal conditions prevail, is an 
open question. 

The vagus is the principal afferent respiratory nerve. The 


efferent nerves are tlie phrenics, intercostals, and others supply- 
ing the various muscles used in moving the chest-walls, etc. 

The respiratory center is automatic, but its action is sus- 
ceptible of modification through afferent influences taking a 
variety of paths.- The respiratory, vaso-motor, and cardio- 
inhibitory centers seem to act somewhat in concert. 

Blood-pressure is being constantly modified by the respira- 
tory act, rising with inspiration and sinking with expiration. 
In some animals the heart-beat also varies with these phases 
of respiration, becoming slow and irregular during expiration. 
Into the causation of these changes both mechanical and nerv- 
ous factors enter, and make a very complex mesh, which we 
can at present but imperfectly unravel. When the access of 
air to the tissues is prevented, a series of stages of respiratory 
activity and decline, accompanied by pronounced changes in 
the vascular system, are passed through, known as asphyxia. 

Three stages are distinguishable: one of dyspnoea, one of 
convulsions, and one of exhaustion — while at the same time 
there is a rise of blood-pressure during the first two, and a 
decline during the third, accompanied by marked alterations in 
the cardiac rhythm. 


As has been intimated from time to time, thus far, as a 
result of the metabolism of the tissues, certain products require 
constant removal from the blood to prevent poisonous effects. 
These substances are in all probability much more numerous 
than physiological chemistry has as yet distinctly recognized 
or, at all events, isolated. Quantitatively considered, the most 
important are carbonic anhydride, water, urea, and, of less im- 
portance, perhaps, certain salts. 

In many invertebrates and in all vertebrates several organs 
take part in this work of elimination of waste products or puri- 
fication of the blood, one set of which — the respiratory — we 
have just studied ; and we now continue the consideration of 
the subject of excretion,' this term being reserved for the pro- 
cess of separating harmful products from the blood and dis- 
charging them from the body. 

We strongly recommend the student to make the study of 
excretion comparative in the sense of noting how one organ 
engaged in the process supplements another. A clear under* 


standing of this relation even to details makes the practice of 
medicine more scientific and practically effective, and gives 
physiology greater breadth. 

The skin has a triple function : it is protective, excretory, 
sensory, and, we may add, nutritive (absorptive) and respira- 
tory, especially in some groups of animals. 

As a sensory organ, the skin will receive attention later. 
Protective Function of the Skin. — Comparative. — Among many 
groups of iji vertebrates the principal use of the exterior cover- 
ing of the body is manifestly protection. Among these forms, 
an internal skeleton being absent, the exo-skeleton is developed 
externally, and serves not only for protection, but for the at- 
tachment of muscles, as seen in crustaceans and insects. But 
this part of the subject is too large for detailed treatment in 

such a work as this. Turning to 
the vertebrates, we see scales, 
bony plates, feathers, spines, hair, 
etc., most of them to be regarded 
as modifications of the epidermis, 
always useful, and frequently also 

Primitive man was probably 
much more hirsute than his mod- 

FlG. 321. 

Fig. 321.— Sudoriparous glands. 1 x 20 (After Sappey.) 1, 1, epidermis ; 2, 2, mucous layer ; 

3, 3, papillae ; 4, 4. derma ; 5, 5, subcutaneous areolar tissue ; 6, 6, 6, 6. sudoriparous 

glands ; 7, 7, adipose vesicles ; 8. S. excretory ducts in derma ; 9, 9, excretory ducts divided. 
Fig. 322.— Portion of skin of palm of hand about one-half an inch (12'7 mm.) square. 1x4. 

(After Sappey.) 1, 1, 1, 1, openings of sudoriferous ducts ; 2, 2, 2, 2, grooves between 

papillae of skin. 

ern representative ; and, though the human subject is at pres- 
ent provided with a skin in which protective functions are at 
their lowest, still the epidermis does serve such a purpose, as 



all have some time realized when it has been accidentally re- 
moved by blistering, etc. 

Taking the structure of the skin of man as representing that 
of mammals generally, certain points claim attention from the 

physiologist. Its elastic- 
ity, the failure of which 
in old age accounts for 
wrinkles ; its epidermal 
covering, made up of 
niimerous layers of cells ; 
its coiled and spiral- 
ly twisted sudoriferous 
glands, permitting of 
movements of the skin 
without harm to these 
structures ; its hair-folli- 
cles and associated seba- 
ceous glands, the fatty 
secretion of which keeps 
the hair and the skin gen- 
erally soft and pliable. 

The muscles of the 
skin, which either move 
it as a whole or erect in- 
dividual hairs, play an 
important part in modi- 
fying expression, well 
seen in the whole canine 
tribe and many others. 

There are several mod- 
ifications of the sebaceous 
glands that furnish high- 
ly odoriferous secretions, 
as in the civet cat, the 
skunk, the musk - deer, 
and many lower verte- 
brates. In some, these 
are protective (skunk) ; 
in others, though they 
may not be agreeable to 
the senses of man, they are doubtless attractive to the females 
of the same tribe, and are to be regarded as important in 
" sexual selection." being: often confined to the males alone. 

Fig. 333.— Hair and hair-follicle (after Sappey). 1, 
root of hair : 2. bulb of hair ; 3, internal root- 
sheath ; 5, membrane of hair-follicle ; 6, external 
membrane of follicle ; 7, 7, muscular bands at- 
tached to Eolhcle ; 8, 8, extremities of bands pass- 
ing to skin ; 9, compound sebaceous gland, with 
duct (10) opening into upper third of follicle ; 11, 
simple sebaceous gland ; 18, opening of hair-fol- 


Ear-wax and the Meibomian secretion are the work of modi- 
fied sebaceous glands ; as also the oil-glands so highly developed 
in birds, especially aquatic forms, of which these creatures 
make great use in preserving their feathers from wetting. 

The Excretory Function of the Skin. 

Sweating in man has been studied by inclosing the greater 
part of the body or a single limb in a caoutchouc or Other form 
of impermeable covering and exposing the subject to various 
degrees of heat ; but, apart from errors in collecting, weighing, 
etc., such sweating must be regarded as somewhat abnormal. 

It is clear, however, that the quantity of matter discharged 
through the "akin is large — greater than by the lungs (about as 
7 to 11), though the amount is very variable, depending on 
the degree of activity of other related excreting organs, as the 
lungs and kidneys, and largely upon the temperature as a 
physical condition. 

When the watery vapor is carried off, before it can condense, 
the perspiration is said to be insensible j when small droplets 
become visible, sensible. As to whether the one or the other 
is predominant will, of course, depend on the rapidity of re- 
newal of the air, its humidity, and its temperature. Apart 
from the temperature, the amount of sweat is influenced by the 
quality and quantity of food and, especially, of drink taken, 
the amount of exercise, and psychic conditions ; not to speak 
of the effect of drugs, poisons, or disease. 

Perspiration in man is a clear fluid, mostly colorless, with 
a characteristic odor, devoid of morphological elements (except 
epidermal scales), and alkaline in reaction. It may be acid 
from the admixture of the secretion of the sebaceous glands. 

Its solids (less than 3 per cent) consist of sodium salts, 
mostly chlorides, cholesterin, neutral fats, and traces of urea. 
The acids of the sweat belong to the fatty series (acetic, buty- 
ric, formic, propionic, caprylic, caproic, etc.). 

FathologicaL — The sweat may contain blood, proteids, abun- 
dance of urea (in cholera), uric acid, oxalates, sugar, lactic acid, 
bile, indigo and other pigments. Many medicines are elimi- 
nated in part through the skin. 

Respiration by the Skin. — Comparative. — In reptiles and batra- 
chians, with smooth, moist skin, the respiratory functions of 
this organ are of great importance; hence these animals can 
live long under water. 


It is estimated ttat in the frog the greater part of the car- 
honic anhydride of the body-waste is eliminated by the skin. 
Certainly frogs can live for days immersed in a tank supplied 
with running water ; and it is a significant fact that in this 
animal the vessel that gives rise to the pulmonary artery sup- 
plies also a cutaneous branch. • 

The respiratory capacity of the skin in man and most mam- 
mals is comparatively small under ordinary circumstances. 
The amount of carbonic anhydride thus eliminated in twenty- 
four hours in man is estimated at not more than 10 grammes. 
It varies greatly, however, with temperature, exercise, etc. 

The skin is highly vascular in mammals, and its importance 
as a heat regulator is thus very great. 

When an animal is varnished over, its temperature rapidly 
falls, though heat production is in excess. From the fact that 
life may be prolonged by diminishing loss of heat through 
wrapping up the animal in cotton-wool, it is inferred that 
depression of the temperature is, at all events, one of the causes 
of death. Though the subject is obscure, it is likely that the 
retention of poisonous products so acts as to derange metabo- 
lism, as well as poison directly, which might thus lead to the 
disorganization of the machinery of life to the point of disrup- 
tion or death. It is also possible that the reduction of the tem- 
perature from dilatation of the cutaneous vessels may be so 
great that the animal is cooled below that point at which the 
vital functions can continue. 

The Excretion op Perspiration. 

In secretion in the wider sense we find usually certain nerv- 
ous and vascular effects associated. The vessels supplying the 
gland are dilated during the most active phase, and at the same 
time nervous impulses are conveyed to the secreting cells which 
stimulate them to action. There is a certain proportion of 
water given off by transpiration ; but the sweat, as a whole, 
even the major part of the water, is a genuine secretion, the 
result of the metabolism of the cells. 

Certain experimental facts deserve consideration in this con- 
nection : 1. If, in the cat, the sciatic nerve be divided and its 
distal end stimulated, even when the vessels of the leg are liga- 
tured, the corresponding foot sweats. 3. The vessels being un- 
touched and atropin injected into the blood, no sweating occurs 
on stimulation of the nerve, though the vessels of the foot 


dilate. 3. If a kitten witli divided sciatic, and as a consequence 
dilated blood-vessels in the corresponding limb, be placed in a 
warm oven, the other feet will sweat, while the one the nerves 
going to which have been divided remains dry. 4. Perspira- 
tion will take place in a cat that has just died under the cir- 
cumstances mentioned in 1. From these experiments it is 
clear that nervous influences alone, in the absence of any vas- 
cular changes, or in the total deprivation of blood, suffice to 
induce the secretion of perspiration. 

If the central stump of the divided sciatic be stimulated, 
sweating of the other limbs follows, showing that perspiration 
may be a reflex act. It is found that stimulation of the periph- 
eral end of the divided cervical sympathetic leads to sweating 
on the corresponding side of the face. 

Human Physiology. — Certain nerves (e. g., the cervical sym- 
pathetic) have been stimulated with results similar to those 
obtained in other animals. We think these experiments and 
certain pathological phenomena, to be presently mentioned, of 
importance beyond their immediate application. They seem to 
show the influence of nerves over vital processes in the clearest 
way, and render it probable that this is the essential element in 
the highest vertebrates, and not the blood-supply, which, though 
important, is subsidiary. The path of the sweat-nerves is 
somewhat similar to that of the vaso-motor fibers, running 
mostly in the sympathetic in some part of their course. 
Whether there is a dominant center in the medulla and subor- 
dinate ones in the cord is a matter of uncertainty ; though, that 
the cerebrum can exercise a powerful influence over the sudor- 
ific glands is evident from the effect of emotions. 

Certain drugs seem to act on the centers through the blood ; 
others on either the nerve terminals or the gland-cells them- 
selves. It is true that some of these will induce sweating after 
the nerves have been divided, though conclusions as to the nor- 
mal action of a part from such experiments must be drawn with 
the greatest caution. In our opinion they are rather suggest- 
ive than demonstrative in themselves, and the views we enter- 
tain of normal function should be formed from a consideration 
of all the eA^idence rather than that from a single experiment, 
however -striking in itself. 

Sweating during dyspnoea and from fear, when the cutane- 
ous surfaces are pale, as well as in the moribund, shows also 
the independent influence over the sudorific glands of the nerv- 
ous system. Heat induces sweating by acting both reflexly and 



directly on the sweat-centers we may suppose. Unilateral 
sweating is known as a pathological as well as experimental 
phenomenon. Perspiration may be either increased or dimin- 
ished in paralyzed limhs, according to circumstances. It is 
possible that there is a paralytic secretion of sweat as of saliva. 
The subject is very intricate and will be referred to again on 
account of the light it throws on metabolic processes generally. 

Absorption by the skin in man and other mammals is, under 
natural conditions probably very slight, as would be expected 
when it is borne in mind that the true skin is covered by sev- 
eral layers of cells, the outer of which are hardened. 

Ointments may unquestionably be forced in by rubbing; 
and perhaps absorption may take place when an animal's tis- 
sues are starving, and food can not be made available through 
the usual channels. It is certain that abraded surfaces are a 
source of danger, from affording a means of entrance for dis- 
ease-producing substances or for germs. 

Comparative. — It is usually stated in works on physiology 
that the horse sweats profusely, the ox less so ; the pig in the 
snout ; and the dog, cat, rabbit, rat, and mouse, either not at all 
or in the feet (between the toes) only. That a closer observa- 
tion of these animals will convince any one that the latter 
statements are incorrect, we have no doubt. These animals, it 
is true, do not perspire sensibly to any great extent; but to 
maintain that their skin has no excretory function is an error. 

Summary. — The skin of the mammal has protective, sensory, 
respiratory, and excretory functions. The respiratory are in- 
significant under ordinary circumstances in this group, though 
well marked in reptiles and especially in batrachians (frog, 
menobranchus). Sweating is probably dependent on the action 
of centers situated in the brain and spinal cord, through nerves 
that run generally in sympathetic tracts during some part of 
their course. While the function of sweating may go on inde- 
pendently of abundant blood-supply, it is usually associated 
with increased vascularity. , 

Sweat contains a very small quantity of solids, is alkaline 
in reaction when pure, but liable to be acid from the admixture 
of sebaceous matter that has undergone decomposition. Sebum 
consists chiefly of olein, palmitin, soaps, cholesterin, and ex- 
tractives of little known composition. The salty taste of the 
perspiration is due chiefly to sodium chloride, and its smell to 
volatile fatty acids ; especially is this so of the sweat of certain 
parts of the body of man and other mammals. 



The functional activity of tlie skin varies with the tempera- 
ture, moisture, etc., of the air and certain internal conditions ; 
especially is it important to remember that it is one of a series 
of excretory organs which act in harmony to eliminate the 
waste of the body, so that when' one functions more the other 
may and usually does function less. 

The protective function of the skin and its modified epithe- 
lium (hair, horns, nails, feathers, etc.) is in man slight, but very 
important in many other vertebrates, among which provision 
against undue loss of temperature is one of the most constant- 
ly operative, and enables a vast number of groups of animals 
to adapt successfully to their varying surroundings. 


The kidney in man and other mammals may be described as 
a very complex arrangement of tubes lined with many different 
forms of secreting cells, sur- 
rounded by a great mesh- 
work of capillaries, boimd 
together by connective tis- 
sue, the quantity varying 
with the animal, and the 
whole inclosed in a capsule. 
The organ is well supplied 
with lymphatics and nerves. 
Though the tubes are so 
complex, the kidney may be 
divided into zones Avhich 
contain mostly but one kind 
of tubule. 

Comparative. — Among the 
lowest forms, the Infusori- 
ans and CaJenterates, ex- 
cretory organs have not 
been definitely traced. In 
the Vermes, organs known 
as nepliridia (segmental or- 
gans, see Figs. 253, 257) are 

T , , ,, J. J? Fia. 344.— Vertical section of Icidney (after Sap- 

Supposed to act the part or pey). l, l, 2, a, 3, 3, 3, 4, 4, 4, 4, pyramids of 

4.r I • i • i? 1 • Malpighi : 5, 5, 5, 5, 5, 5, apices of pyramids, 

the kidney m some fashion. surrounded by caUces ; C, 6, columns of Ber: 

These are long, often coiled 'iVr. p"'" °' '"*°'^ '' '' ""P"' '^''""''^ "' 



tubes lined with cells, and with an internal, cilated, funnel- 
shaped extremity opening into the body cavity. In such cnis- 

FiQ. 325.— structure of kidney (after Landois). I. Blood-vessels and tubes (semi-diagram- 
matic). A. Capillaries of cortical substance. B. Capillaries of medullary substance. 
1, artery penetrating Malpighian body ; 2, vein emerging from a Malpighian body ; n- 
arteriolte rectje ; C, venEe rectee ; r, V, interlobular veins ; S, stellate veins ; /, /, caj), 
sules of MUller ; X. X, convoluted tubes ; T, T, T, tubes of Henle ; N, N, N,N, commum- 
eating tubes ; O, O, straight tubes ; O, opening into pelvis of kidney. U. Malpighian 
body. A, artery ; E, vein ; C, capillaries ; K, epithelium of capsule ; H, beginning or 
convoluted tube. in. Rodded cells from convoluted tube. 1, view from surface ; 8, siae 
view (G. granular zone). IV. Cells lining tubes of Henle. V. Cells lining communicating 
tubes. VI. Section of straight tube. 



taceans as tlio crayfish the green gland is supposed to repre- 
sent a kidney. It does not open into tlie body cavity like the 
preceding and the following form of the organ. It is well sup- 
plied with capillaries. The organ of Bojanus (Fig. 30G) is the 

FiQ. 326— Blood-vessels of Malpighian bodies and convoluted tubes of kidney (after Sappey). 
1, 1, Malpiprhian bodies surrounded by capsules ; 2, 2, 2, convoluted tubes connected with 
Malpighian bodies ; 3, artery branching to go to Malpighian bodies ; 4, 4, 4, branches of 
artery ; 6, 6, Malpighian bodies from which a portion of capsules has been removed ; 
7. 7, 7, vessels passing out of Malpighian bodies ; 8, vessel, branches of which (9) pass to 
capillary plexus (10). 

main excretory channel in many groups of moUusks. In in- 
sects the long, coiled Malpighian tubules, which open into the 
intestine, are believed to secrete both bile and uric acid. 

Among vertebrates, till the reptiles are reached, the kidney 
is a persistent Wolfifian body, hence its more simple form. 




In most fishes the kidney is 
a very elongated organ, though 
in the lowest it consists of little 
more than tubules, coiling but 
slightly, ending by one extrem- 
ity in a glomerulus and by the 
other opening into a long com- 
mon efferent tube or duct. The 
glomerulus is, however, pecul- 
iar to the vertebrate kidney. 
The graded complexity in ar- 
rangement, etc., of the tubes is 
*^ represented well in the figure 
below. It is a significant fact 
that the kidney of the human 
subject is lobulated in the em- 
bryo, which condition is persist- 
ent in some mammals (rumi- 
nants, etc.). 

As the lungs are the organs 
employed especially for the 
elimination of carbonic anhy- 
dride, so the kidneys are above 
all others the excretors of the 
niti'pgenous waste products of 
the body chiefly in the form of 
uric acid or urea. Before treat- 
ing of secretion by the kidney 
it will be well to examine into the physical and chemical prop- 
erties of urine with some detail, especially on account of its 
great importance in the diagnosis of disease. 


Fig. 337.— Diagrammatic representation of 
distribution of tubules of kidney (after 
Huxley). C, cortical region ; B, bound- 
ary zone, containing large part of Hen- 
le's loops : P, papillary zone, in which 
are the main outflow tubules. 

Urine considered Physically and Chemically. 

Urine is naturally a fluid of very variable composition, espe-. 
cially regarded quantitatively — a fact to be borne in mind in 
considering all statements of the constitution of this fluid. 

Specific Gravity. — Urine must needs be heavier than water, on 
account of the large variety of solids it contains. The average 
specific gravity of the urine for the twenty-four hours is 1015 
to 1020. It is lowest in the morning and varies greatly with 
the quantity and kind of food eaten, the activity of the lungs 
and especially of the skin, with emotions, etc. 



Color. — A light straw color, which is also very variable, 
being increased in depth either by the presence of an excess of 
pigment or a diminution of water. There are probably several 
pigments, among which occur urobilin, derived probably from 
bile pigment ; urochrome, becoming red on oxidation ; and 
indican, which may be oxidized to indigo. 

The reaetioa of human urine is acid, owing to acid salts, espe- 
cially acid sodium phosphate (NaHjPOi). There is usually but 
a trifling quantity, if any, of free acid in the urine when 
secreted. The acidity diminishes after meals, and the urine 
may be neutral or alkaline when the food is wholly vegetable, 
or unduly acid when the diet is entirely fleshy. 

ftuantity. — Usually about 1,500 c.c. or from 50 to 53 ounces 
(two pints) in twenty -four hours. This is, of course, like the 
specific gravity, highly variable, and frequently they run par- 
allel with each other. 

The following tabular statement will prove useful for refer- 

Quantitative Estimation of the Constituents of the Urine for 
Twenty-four Hours {after Parkes). 

By an average 
man of 65 kilos. 

Per 1 kUo of 
body weight. 




















Total solids 


Urea • 

Uric acid 


Hippuric aoid ... 



Pigment, etc 

Sulphuric acid 


Phosphoric a<iid . . .... 







Attention is directed more particularly to the preponderance 
among the solids of urea, and sodium chloride, for tlie latter is 
the form in which a large part of the sodium reappears. We 
may say that in round numbers about 35 grammes or 500 grains 
(2 to 3 per cent) of urea are excreted daily. 

Nitrogenous CrystaUine Bodies. — These are the derivatives of 
the metabolism of the body, and not in any appreciable degree 
drawn from the food itself. Besides urea, and of much less 


importance, occurring in small quantities, are bodies that may 
be regarded as less oxidized forms of nitrogenous metabolism, 
such as creatinin, xanthin, hypoxanthin (sarkin), hippuric acid, 

ammonium oxalurate, and urea, CO -j ^ji'. The latter was 

first prepared artificially from ammonium cyanate, ^it [■ 0, 
with which it is isomeric. 

When air has free access to urine for some time in a warm 
room, the urea becomes ammonium carbonate by hydration, 
probably owing to the influence of micro-organisms, thus: 
CO (NHs)2 + 2 HsO = (NH4)3 COj; hence the strong ammonical 
smell of old urine, urinals, etc. 

Uric acid (C6H4N4O3) occurs sparingly (see table), combined 
with sodium and ammonium chiefly as acid salts. Since these 
salts are not so soluble in cold as in warm water, they often 
fall as a sediment in the vessel in which the urine stands, and 
present a brick-red or fawn color. 

Uric acid is itself rather insoluble in cold water or hydro- 
chloric acid, though soluble in alkalies ; hence it may be 
obtained by adding hydrochloric acid to the urine in the cold. 
When it is in excess it may separate out on standing, forming 
characteristic colored (dark-red) crystals, adhering to the sides 
of the vessel, floating on the top of the urine, or as a sediment 
at the bottom (like red-pepper grains). 

Non-nitrogenous Organic Bodies. — Whether traces of sugar are 
normal in urine is as yet undetermined. Certain acids occur, 
at least frequently, in small quantities, and combined mostly 
with bases. Among these are lactic, formic, oxalic, succinic, 
etc. A series of well-known aromatic bodies occurs in urine, 
especially in that of the horse, cow, etc. These are phenol, 
cresol, pyrocatechin, which occur not free, but united with sul- 
phuric acid. J 

Inorganic Salts. — These are mostly in simple solution, in urine,' 
and not, as in some other fluids of the body, united with pro- 
teid bodies. The salts are chlorides, phosphates, sulphates, 
nitrates, and carbonates, the first three being the most abun- 
dant ; the bases being sodium, potassium, calcium, magnesium. 
Since the earthy salts can not remain in solution in an alkaline 
fluid, they are usually found as a sediment when the urine loses 
its acid reaction. The phosphates are to be traced to the food, 
to the phosphorus of proteids, and to phosphorized fats (leci- 
thin). The sulphates are derived from those of the food and 
from the sulphur of the proteids of the body. So much of the 


carbonates as is not derived directly from a corresponding sup- 
ply in tlie food may be traced to the oxidation of certain or- 
ganic salts, as citrates, tartrates, etc. 

Doubtless many bodies appear either regularly or occasion- 
ally in urine that have so far escaped detection, which are, like 
the poisonous exhalations of the lungs, not the less important 
because unknown to science. 

Abnormal TTrine. — There is not a substance in the urine that 
does not vary under disease, while the possible additions act- 
ually known are legion. These may be derived either from 
the blood or from the kidneys and other parts of the urinary 
tract. The kidneys seem to take upon themselves more readily 
than any other organ the duty of eliminating foreign matters 
from the body. But this aspect of the subject is too wide for 
detailed consideration in this work. 

The student of medicine should be thoroughly familiar with 
the urine in its normal condition before he enters upon the 
examination of the variations produced by disease. This is 
not difficult, and much of it may be carried out with but a 
meager supply of apparatus. For this purpose, however, we 
recommend some work devoted to the chemical and micro- 
scopic study of the urine. 

It greatly assists to remember a few points in regard to 
solubilities. Fropi a physiological point of view, the urine and 
its variations, as the result of changes in the organism, may be 
observed with advantage in one's own person — e. g., the influ- 
ence of food and drink, temperature, emotions, etc. 

Comparative. — The urine of most vertebrates is of higher spe- 
cific gravity than that of man. In fishes, reptiles, and birds, 
uric acid replaces urea, and is very abundant. In these animals 
most of this substance is white. The urine is passed with the 
faeces. Among mammals the urine of the carnivora is strongly 
acid, perhaps owing in great part to the flesh on which they 
feed ; and abounds in phosphates and, in some instances, sul- 
phates. The urine is so concentrated in some cases tha^t we 
have known urea nitrate to crystallize out on the addition of 
nitric acid without requiring condensation. 

The urine of the herbivora is alkaline, and abounds in salts 
of calcium, especially carbonates. It is also of high specific 
gravity, and grows rapidly dark in color when passed, owing 
probably to the presence of the aromatic compounds referred 
to above, derived from the food. In certain groups of inverte- 
brates uric acid seems to be a normal excretion. 


The Secretion of Ueine. 

Among experimental facts from which important conclu- 
sions have been drawn are the following (when blood-pressure 
within the kidney is referred to, it will be understood that the 
glomeruli are meant) : 1. Section of the spinal cord, which 
greatly lowers the general blood-pressure, is followed by dimi- 
nution or total arrest of the secretion of urine. 2. Section of 
the renal nerves, and to a less extent of the splanchnics de- 
creases the flow of urine. 3. Stimulation of the spinal cord 
after section of the above nerves (which raises the blood-press- 
ure in the kidney by elevating the general blood-pressure) in- 
creases the flow of urine. 4. Certain diuretics increase the 
blood-pressure, either generally or in the kidney, while others 
act on the renal epithelium, apparently independently of blood- 

By means of apparatus adapted to register the changes of 
volume the kidney undergoes, it is found that the kidqey not 
only responds to every general change in blood-pressure, but 


Fig. 338. — BP, blood-pressure curve ; K, curve of the volume of the kidney ; T, time-curve, 
intervals indicate a quarter of a minute ; A, abscissa (Stirling, after Roy). 

to each heart-beat — that is, its volume varies momentarily. 
This shows how sensitive it is to variations in blood-pressure. 

From the above and other experiments it has been concluded 
that the secretion of urine is largely dependent on blood-press- 
ure. Until very recently certain experiments (of Nussbaum) 
were considered as favoring the view that the activity of the 
glomeruli was of a wholly or greatly different character from 
that of the tubules. In the amphibia (frog, newt, etc.) there is 
a double renal blood-supply. The glomeruli derive their blood 
from the renal artery, and the tubules from the renal-portal 
system. The vein returning the blood from the lower extrem- 
ity divides (Fig. 231) at the upper part of the thigh into two 
branches, one of which, entering the kidney, breaks up into 


capillaries around the tubules, whicli inosculate to some extent 
with the vessels emerging from the glomeruli. It was found 
that when certain substances were injected into the blood they 
no longer appeared in the urine after the renal artery had been 
tied, from which it was concluded that they were secreted only 
by the glomeruli, and that the blood of the renal-portal vein 
did not find access to the glomeruli. This conclusion was a 
pretty bold leap, but there was some show of reason for it. 
More recently, however, these experiments have been demon- 
strated to be, to a certain extent, unreliable, and that the pas- 
sage of blood from the venous capillaries backward can really 
take place, to some extent, after a time. 

Theories regarding the secretion of urine may be divided 
into those that are almost wholly mechanical, partly mechani- 
cal, and purely secretory : 1. To the first class belongs that of 
Ludwig, which teaches that very dilute urine is separated from 
the blood in the glomeruli, and by a process of endosmosis and 
absorption of water by the tubular capillaries is gradually 
concentrated to the normal. 2. As an example of the second 
class is that of Bowman, who maintained that the greater part 
of the water and some of the more soluble and diffusible salts are 
separated by the glomeruli but the characteristic constituents 
of the urine by the epithelium of the renal tubules. 3. As an ex- 
ample of the third is the theory of Heidenhain, who attributed 
little to blood-pressure in itself, and much, if not the whole, to 
the secreting activity of the epithelium of the tubules more par- 
ticularly. This physiologist showed that^ while ligature of a 
vein raised the blood-pressure within a glomerulus, it was not 
followed by any increase in the quantity of the secretion, but 
by its actual arrest. He also showed that injection of a colored 
substance (sodium sulphindigodate) into the blood, after the 
pressure had been greatly lowered by section of the spinal 
cord, led to its appearance in the urine ; and microscopic exam- 
ination showed that it had passed through the epithelial cells 
of the tubules, not of the glomeruli. 

It is found, however, that after the removal of a ligature 
applied to the renal artery the urine is albuminous, showing 
that the cells have been plainly injured by the operation ; hence 
Heidenhain's experiment described above is not valid against 
the blood-pressure theory. Moreover, too much must not be 
inferred from the action of foreign substances under the ab- 
normal conditions of such an experiment. While some physi- 
ologists claim that the glomeruli are filtering mechanisms, they 


explain that filtration is not to be understood in its ordinary- 
laboratory acceptation, but that the glomeruli discriminate as 
to what they allow to pass, yet they in no way explain how 
this is done. They make the whole process depend on blood- 
pressure, and attribute little special action to the flat epithe- 
lium of the Malpighian capsules. 

Though we can not admit the full force of Heidenhain's ex- 
periments as he interprets them, we still believe that his views 
are most in harmony with the general laws of biology and the 
special facts of renal secretion. Recently, after a repetition of 
Nussbaiim's experiments, and the institution of others, it has 
been rendered clear that the mechanical theory of the work of 
the kidney can not hold, even of the glomeruli, which are 
shown to be, as we should have expected, true secreting organs. 
Now, there can be no doubt that blood-pressure is a most im- 
portant determining condition here as in other secreting pro- 
cesses, in the mammal at all events ; but whether of itself or 
because of the influence it has on the rapidity of blood-flow, it 
is diflicult to determine ; or rather whether solely to the latter, 
for that the constant supply of, fresh blood is a regular con- 
dition of normal secretion there can be no doubt. Further, it 
seems probable that blood-pressure has more to do with the 
secretion of water than any other constituent of urine. But 
we maintain that it should be called a genuine secretion, and 
that nothing is gained by using the term " filtration "—on the 
contrary, that it is misleading, and tends to divert attention 
from the real though often hidden nature of vital processes. 
The facts of disease and the evidence of therapeutics, we think, 
all favor such a view of the work of the kidneys. 

Nerves having an influence over the secretion of urine simi- 
lar to those acting on the digestive glands have not yet been 
determined. The powerful influence of emotion, especially in 
those of unstable nervous system, over the secretion of urine 
shows that there must be nervous channels through which the 
nerve-centers act on the kidneys ; though whether the results 
are not wholly dependent upon vaso-motor efEects may be con- 
sidered as an open question by many. We think such a view 
improbable in the highest degree. The most recent investiga- 
tions would seem to show that the vaso-motor fibers run in the 
dorsal nerves, especially 'the eleventh, twelfth, and thirteenth, 
and that of these the vaSo-constrictors are the best developed. 

Pathological. — When the kidneys are excised, the ureters 
ligatured, or when the former are so diseased as to be inca- 


pable of performing their functions, death is the result, being 
preceded by marked depression of the brain-centers passing 
into coma. Exactly which of the retained products brings 
about these results is not known. They are likely due to sev- 
eral, and it impresses on the mind the importance of those 
processes by which the constantly accumulating waste is elimi- 
nated. Uric acid when not removed from the blood and tissues 
is supposed to be the exciting cause of gout. An excess in the 
form of urates retained or deposited in certain parts, especially 
the joints, is frequently at all events an accompaniment of this 

The Expulsion of Urine. 

We now present in concise form certain facts on which to 
base opinions as to the nature of the processes by which the 
bladder is emptied. 

It will be borne in mind that the secretion of urine is con- 
stant, though of course very variable; that the urine is con- 
veyed in minute quantities by rhythmically contractile tubes 
(ureters) which open into the bladder obliquely ; and that the 
bladder itself is highly muscular, the cells being arranged both 
circularly and obliquely, with a special accumulation of the 
circular fibers around the neck of the organ to form the sphinc- 
ter vesiccR. 

1. It is found that the pressure which the sphincter of the 
bladder can withstand in the dead is much less (about one 
third) than in the living subject. 3. We are conscious of being 
able to empty the bladder, whether it contains much or little 
fluid. 3i We are equally coi^scious of an urgency to evacuation 
of the vesical contents, according to the fullness of the organ, i 
the quality of the urine, and a variety of other conditions. 

4. Emotions may either retard or render micturition urgent. 

5. In a dog, in which the cord has been divided in the dorsal 
region some months previously, micturition may be induced 
reflexly, as by sponging the anus. 6. In the paralyzed there 
may be retention or dribbling of urine. 7. In cases of urethral 
obstruction from a calculus, stricture, etc., there may be excess- 
ive activity of the muscular tissue of the bladder-walls. 8. 
Evacuation of the bladder may occur in the absence of con- 
sciousness (sleep). 

The most obvious conclusions from these facts are that — 1. 
The urine finds its way to the bladder partly through muscular 
(peristaltic) contractions of the ureters, partly through gravity. 


in man at all events, and partly from the pressure within the 
tubnles of the kidneys themselves. 2. The evacuation of urine 
may take place independently of the will (see 8), and is a reflex 
(5) act. 3. Micturition may be initiated by the will, which is 
usually the case, when by the action of the abdominal muscles 
a little urine is squeezed into the urethra, upon which afferent 
impulses set up contractions of the bladder by acting on the 
detrusor center of the cord and at the same time inhibit the 
center presiding over the sphincter (if such there be), permit- 
ting of its relaxation. 4. Emotions seem to interfere with the 
ordinary control of the brain-centers over those in the spinal 
cord. 5. It may be assumed that the normal tone of the 
sphincter of the bladder is maintained reflexly by the spinal 
cord. The unwonted muscular contraction associated with an 
obstruction to the outflow of urine may be in part of nervous 
origin, but is also, in all probability, owing in some degree to 
the muscle-cells resuming an independent contractility, due to 
what we recognize as the principle of reversion. The same is 
seen in the heart, ureters, and similar structures. 

Pathological. — There may be incontinence of urine from pa- 
ralysis, the cerebral centers being unable to control those in 
the spinal cord. Dribbling of urine may be due to retention in 
the first instance, the tone of the sphincter being finally over- 
come, owing to increase of pressure within the bladder. Over- 
distention of the bladder may arise in consequence of lack of 
tone in the muscular walls, though this is rare. Strangury is 
due to excessive action of the walls of the bladder and the 
sphincter, brought about reflexly, when the organ is unduly 
irritable, as in inflammaition, after, the abuse of certain drugs 
(cantharides), etc. 

Comparative. — In man the last drops of urine are expelled by 
the action of the bulbo-cavernosus muscle and perhaps some 
others. In the dog and many other animals the regulated and 
voluntary use of this muscle, marked in a high degree, produces 
that interrupted flow so characteristic of the micturition of 
these animals. 

Summary. — Urine is in man a fluid of specific gravity 1015 
to 1020, acid in reaction, pale yellow in color, and containing 
certain salts, pigments, and nitrogenous bodies. The chief of 
the latter is urea, which is excreted daily to the extent of about 
one ounce (500 grains). 

The kidneys and skin especially supplement one another, 
and normally great activity of the one implies lessened ac- 


tivity of the other. This is availed of in the treatment of dis- 

Both the Malpighian capsules and the renal tubules have a 
true secretory function, though the larger part of the water of 
urine is secreted by the former. Blood-pressure is an important 
condition of secretion, though it is likely that this is so chiefly 
because it favors a rapid renewal of the blood circulating 
through the organ. "Whether there are nerves that influence 
secretion directly, as in the case of the skin, is not determined. 

Suppression of th€i renal functions leads to symptoms in 
which the nervous system is recognized as suffering to the 
extent often of coma, ending in death. The urine of most other 
animals is more concentrated than that of man ; this secretion 
in carnivora being acid, and in herbivora alkaline in reaction 
when passed a short time. 

Our information in regard to the kidneys has been derived 
experimentally chiefly from the study of the frog and a few of 
the domesticated mammals, especially the dog ; and as regards 
some points of interest, so far as urine is concerned, from the 
bird (guano), and the horse, ox (aromatic compounds), etc. 
Man's urine has been thoroughly studied ; but the nature of 
the act of renal secretion is in his case a matter of inference 
from the facts of pathology, clinical medicine, therapeutics, etc. 


In the widest sense the term metabolisTn may be conven- 
iently applied to all the numerous changes of a chemical kind, 
resulting from the activity of the protoplasm of any tissue or 
organ. In a more restricted meaning it is confined to changes 
undergone by the food from the time it enters till it leaves the 
body, in so far as these are not the result of obvious mechani- 
cal causes. The sense in which it is employed in the present 
chapter will be plain from the context, though usually we shall 
be concerned with those changes effected in the as yet compara- 
tively unprepared products of digestion, by which they are ele- 
vated to a higher rank and brought some steps nearer to the 
final goal toward which they have been tending from the first. 
As yet our attempts to trace out these steps have been little 
better than the fruitless efforts of a lost traveler to find a road, 
the general direction of which he knows, but the ways by which 
it is reached only the subject of plausible conjecture. But 


any theories that, like a scaffolding, allow of or help to addi- 
tional investigation, and in any way lead out into a clearer 
light, are not without value; and on this principle we shall 
treat the subject, spending but little time in barren fields 
except as they have an interest from the suggestiveness of the 
results, even though negative. 

The Metabolism of the Liver. 

This organ has two well-recogniaed functions : 1. The for- 
mation of bile. 2. The formation of glycogen. 

We have already considered the first, and ascertained how 
little .is positively known. Let us now examine the second. 

Glycogen may be obtained from the liver of mammals, such 
as the rabbit, by rapidly killing the animal, excising the warm 
still living organ, cutting into fine pieces, throwing them into 
boiling water, removing after a few minutes and grinding in a 
mortar and reimmersing in the boiling water ; on now passing 
the latter through a coarse filter a turbid, whitish fluid is ob- 
tained containing the extracted glycogen as proved by giving 
a red color with solution of iodine. The substance may be ob- 
tained as a whitish amorphous powder, having the chemical com- 
position of starch, and has in fact been termed animal starch. 

By appropriate treatment it may be converted into sugar by 
a process of hydration (CeHjoOs + HaO = CeHijOe). 

If, after the death of an animal, the liver be kept at body 
temperature for, say, aa hour, very little glycogen can be recov- 
ered from it, but instead abundance of sugar. These facts sug- 
gest that the sugar present represents the original glycogen, 
and that the conversion has been effected by some ferment, 
which does not act during life, though why not is one of the 
problems ranking with the non-digestion of the stomach by its 
own ferments, etc. 

We have already expressed our doubts as to the justifia- 
bility of resorting to so many " ferments " to explain the facts 
of physiology, and in the present case there is another possible 
view of the matter. It is conceivable that the conversion, 
under these circumstances, of the glycogen into sugar, may be 
an act of the dying protoplasm of the liver-cells ; and there are 
experimental results which tend to strengthen such a view. 

The principal facts as to the storage of glycogen in the liver 
may be briefly stated thus : 

1. Glycogen has been found in the liver of a large number 


of groups of animals including some invertebrates. 2. Among 
mammals it is most abundant when the animal feeds largely 
on carbohydrates. 3. It is found in the liver of the carnivora, 
and in those of omnivora, when feeding exclusively on flesh. 
4. When an animal starves (does not feed), the glycogen grad- 
ually disappears. 5. A fat-diet does not give rise to glycogen. 
6. During early foetal life glycogen is found in all the tissues, 
but later it is restricted more and more to the liver, though 
even in adults it is to be found in various tissues, especially the 
muscles, from which it is almost never absent. 

From the facts the inference is plain4hat glycogen is formed 
from carbohydrate materials ; or, to be rather more cautious, 
that the formation of this substance is dependent on the pres- 
ence of such material in the food. Inasmuch as glycogen oc- 
curs in muscle, it does not follow, from the fact of its presence 
in the liver of carnivorous animals, that it is manufactured 
from proteid substances, though this is not more difficult to 
understand chemically than the formation of fat from this 
source which is well established. 

Starch, it is well known, occurs abundantly in plants, and 
there is no doubt that the sugar often present in abundance has 
starch as its antecedent, and vice versa. Analogy, then, points 
to such a relation between carbohydrate food and glycogen for- 
mation on the one hand, and reconversion of glycogen into 
sugar on the other. And recent investigations tend to show 
that plant metabolism bears a greater resemblance to that of 
animals than was till recently supposed, thus giving greater 
force to the argument from analogy, though this is recognized 
as generally a dangerous one. 

Assuming this relation between food-stuffs and glycogen to 
hold, the question arises. How is the substance formed by the 
liver ? There are three conceivable methods : 1. The liver-cells 
may, we know not how, simply dehydrate the sugar of diges- 
tion as carried to them in the portal blood. 2. The cells may 
manufacture glycogen from their own protoplasm, in which 
process the portal sugar is in some way used. 3. The liver-cells 
may always be engaged in the construction of glycogen as the 
gastric cells of pepsinogen, but the accumulation or removal of 
the substance depends on the character of the food especially ; 
thus, if the latter abounds in carbohydrates, the blood will be 
well supplied with sugar, so that the glycogen need not undergo 
its usual conversion into that substance. None of these views 
has been definitely proved to be the correct one. 



The Uses of Glycogen.— Whether the blood of the hepatic vein 
contains more sugar than that of the portal vein has long been 
a subject of controversy. If the affirmative could be established, 
it would be pretty clear that glycogen stored in the liver-cells 
was transformed into sugar, possibly by a process of hydration. 
But, considering the rapidity of the blood-stream, it is easy to 
understand that a large amount of sugar might be conveyed 

Main verume trunk 

Bight awiole 

Vena cava 

Bepatie eefn- 

Ijymph. gtana 

Portal system 

S'i ^^p'^-^^ 

Sg Blood vessel, tissue cells, 
&„(^m lympti-spaces 

Alimentary tract 

Fie. 329.— Diagram intended to illustrate the general relations of blood and lymph to metab- 
olism (nutrition) ; and the method by wmch the portal, lymphatic, and general venous 
systems are related to the alimentary tract. 

into the general circulation, and yet the blood, whether of the 
hepatic vein or of other parts, contain but a small quantity at 
any one time. The blood is kept of a certain fairly constant 
composition, both by the action of the excreting organs and by 
the withdrawal from it of supplies for the tissues. Moreover, 
that correlation of functional work on which we have already 
insisted, is not to be forgotten. One must not conceive of the 
liver-cells or any others doing their work independently of the 
condition of their fellow cell-units in the organic common- 
wealth. We mean to say that the amount of glycogen trans- 
formed to sugar will depend on a great many circumstances 
outside of the liver itself. Such aspects of the case have been 
rather overlooked. According to another theory, glycogen is 
an intermediate product between sugar and fat, but of this 
there is very little evidence indeed ; and, besides, fat formation 
is otherwise well enough accounted for, though, of course, too 
much stress must not be laid upon such an argument. 

What is the fate of the transformed glycogen ? What be- 
comes of the sugar ? We can answer, negatively, that it is not 


used up in the blood, it is not oxidized there; but by what 
tissues it is used or how it is made available in the economy is 
a subject on which we are profoundly ignorant. The presence 
of so much glycogen in the partially developed tissues of the 
foetus points to its importance, and suggests its being a crude 
material which is laid up to be further elaborated, as in vege- 
tables, by the growing protoplasm. 

Glycogen being so generally present in muscle, its diminu- 
tion running parallel, to some extent at least, with the func- 
tional activity of the tissue, it is clear that there is some im- 
portant purpose served ; but here again we inquire. What ? 

Pathological. — If a point in the medulla oblongata of a rabbit, 
corresponding nearly or completely with the vaso-motor center, 
be punctured, the urine will in a few hours be found aug- 
mented in quantity and containing sugar. 

It is further found that the quantity of the latter bears 
some relation to the diet of the animal, one well fed on carbo- 
hydrates having more sugar in the urine than a fasting animal. 
From these facts it has been concluded that the nervous system 
has lost a customary normal influence over the glycogenic 
function, either directly through the action of the nerves on 
the liver-cells or through the loss of tone arising from injury 
to the vaso-motor center. Poisoning by carbonic oxide and the 
administration of certain drugs also causes sugar to appear in 
the urine. 

The symptoms resulting from puncture of the medulla, etc., 
have been spoken of as " artificial diabetes " — a very objection- 
able term for which artificial glycosuria should be substituted. 
There is a grave and often fatal disease known as diabetes 
mellitus, one of the symptoms of which is the appearance often 
of enormous quantities of grape-sugar in the urine. But all 
attempts to fathom the depths of obscurity which surround this 
malady have been in vain. It would seem that attention has 
been directed too exclusively to the liver. Cases of the disease 
occur in which at the post-mortem examination the liver may 
be perfectly normal in appearance, or either hypereemic or 

It seems to us that it is likely that the disease will be shown 
to be of diverse origins, or certainly not referable to one organ 
solely in most cases. The conclusion that the nervous system 
is greatly concerned, both in directing the glycogenic functions 
of the liver and in the disease in question, seems to be un- 
doubted ; vaso-motor effects, when present, being probably of 



secondary importance. "We doubt, however, if the results of 
the above-mentioned experiment warrants any inferences as to 
the normal glycogenic functions. 

The instructive part about the disease diabetes is the man- 
ner in which the course of events emphasize the importance 
of co-ordination among the vital processes, and the constant 
necessity for regulation of them all by the nervous system. 
Diabetes seems to imply that these processes have escaped this 
normal control and are running riot. 

Metabolism of the Spleen. 

The physiological significance of the peculiar structure of 
this organ, though not yet fully understood, is much plainer 

Fig. 330. — Vertical section of a small superficial portion of human spleen, seen with low power 
(Schafer). 4, peritoneal and fibrous covering ; b, trabeculas ; c, r, Malpighian corpuscles, 
in one of which an artery is seen cut transversely, in the other, longitudinally ; d, mjected 
arterial twigs ; e, spleen-pulp. 

than it was till recently. The student is recommended to look 
carefully into the histology of the spleen, especially the dis- 
tribution of its muscular tissue and the peculiarities of its 
blood-vascular system. It has already been pointed out that 
there is little doubt that leucocytes are manufactured here even 
in the adult, possibly also red cells ; and that the latter are dis- 
integrated, and the resulting substances worked over, possibly 
by this organ itself. This view is rendered probable, not only 
bv microscopic studv of the orgran, but bv a chemical examina- 



tion of the splenic pulp ; for a ferruginous proteid, and numer- 
ous pigments, of a character such as harmonizes with this con- 
ception, are found. 

The fact that the spleen-pulp does not agree in composition 
with either blood or serum : that it abounds in extractives such 

-Thin section of spleen-pulp, highly masniiied, showing mode of origin of a small 
3 of pulp (Schafer^. ?;, vein filled with bloc ' ' '' ' 

Fig. 3.S1.- 

vein in the interstices of pulp (Schafer^. 'v, vein filled with biood-corpuscles, which are in 
continuity with others, bl, filling up interstices of retiform tissue of pulp ; w, wall of 
vein. The shaded bodies among red corpuscles are pale corpuscles. 

as lactic, butyric, formic, and acetic acids, together with inosit, 
xanthin, hypoxanthin, leucin and uric acid — points to its being 

Fig. 332. — Portion of spleen of cat, showing Malpighian (lymphatici corpuscle (after Cadiat). 
A, artery around which corpuscle is placed ; B, meshes of spleen-pulp, injected ; C, artery 
of corpuscle ramifying in lymphatic tissue. The clear space around corpuscle represents 
a lymphatic sinus. 

the seat of a complex metabolism, though neither the changes 
themselves nor their purpose are well understood. 

Nevertheless, it must be admitted that to recognize this was 
a great advance upon the view that the spleen had no impor- 


taut function, and that this was shown by the removal of the 
organ without change in the animal's economy. 

But to believe that there are no such changes, and to have 
clear proof of it, are two different things. As a matter of fact, 
closer study does show that in some animals there are altera- 
tions in the lymphatic glands and bone-marrow, which organs 
are undoubtedly manufacturers of blood-cells. 

These changes are unquestionably compensatory, and that 
other similar ones corresponding to comparatively unknown 
functions of the spleen have not as yet been discovered is owing 
likely to our failures rather than their real absence. We dwell 
for a moment on this, because it illustrates the danger of the 
sort of reasoning that has been applied in the case of this and 
other organs ; and it shows the importance of recognizing the 
force of the general principles of biology, and also the desira- 
bility of refraining from drawing conclusions that are too wide 
for the premises. In every department of physiology it must 
be more and more recognized that what is true of one group 
of animals is not necessarily true of another, or even of other 
individuals, though the differences in the latter case are of 
course usually less marked. We have referred to this be- 
fore, and shall do so again, for it is as yet but too little con- 

Examinations of the spleen, carried out by means of the on- 
cograph, as in the case of the kidney, reveal the following facts: 
1. The spleen undergoes slight changes in volume, correspond- 
ing to the respiratory undulations of blood-pressure, but not, as 
with the kidney, to each heart-beat. 2. The spleen experiences 
rhythmic variations in size, independent of the general blood- 
pressure. It will be borne in mind that the splenic arteries end 
in capillaries, but that some of the arterial blood finds its way 
possibly from the capillaries into the splenic pulp, from which 
it is taken up by veins beginning in this tissue. 

It is highly probable, then, that these movements serve to 
propel the blood that has found its way into the pulp-tissue on- 
ward into the veins ; and it is not to be forgotten that among 
large groups of invertebrates, in which capillaries are wanting, 
a not very unlike method of carrying on the general circula- 
tion is found ; at the same time, we may suppose that such an 
arrangement of blood-supply and removal would not be un- 
favorable to splenic metabolism. 

There is one fact in the metabolism of the spleen that de- 
serves special notice, though we can not indicate all its bear- 


ings. Uric acid is found in the spleen, even of herbivorous 
animals, though not in their urine. 

Abscissa of Blood-pressure curve. S seconds intervals. 


Fig, 333.— Tracing of splenic variations in size, taken with the oncoffraph (after Roy). The 
increase in volume is indicated in upper curve by the ascent and the diminution by the 
descent. The tracing below is of the blood-pressure as taken in carotid artery of dog. 
The lower line indicates time markings. 

It is known that this constituent of the urine is increased in 
intermittent fever (ague), in which disease the spleen is often 
greatly enlarged. The vascular engorgement and the height- 
ened metabolism of the spleen seem to be associated ; and the 
fact that the uric-acid diathesis is often intensified if not origi- 
nated by overfeeding, suggests a connection between the spleen 
and the digestive system at all events. Much as there is that 
remains obscure, we think it can not be doubted, on the evi- 
dence furnished, that the spleen must serve some very impor- 
tant purpose in the economy, apart from its relations to the 
blood, noticed in an earlier chapter. 

The dominion of the nervous system over the spleen is evi- 
dent from various facts. The spleen may be diminished in size 
either generally by the stimulation of the vagus or splanchnic 
nerves directly, or reflexly through stimulation of one of the 
afferent nerves ; and, locally, by direct application of the elec- 
trodes to the surface of the organ. Stimulation of the medulla 
itself also leads to contraction of the organ. It would seem 
that not only the arteries but the organ as a whole is main- 
tained in a state of tonic contraction to a certain extent by the 
agency of the nervous system. Not only so, but, if we may 
judge from the analogy of other organs, we may believe that 
its metabolism is directly controlled by the nervous system. 


The Construction of Fat. 

It is a well-known fact that, speaking generally, a diet ricli 
in carbohydrates favors fat formation, both in man and other 
animals ; though it is not to be forgotten that many persons 
seem to be unable to digest such food, or, at all events, to as- 
similate it so as to form fat to any great extent. Persons given 
to excessive fat production are as frequently as not sparing 
users of fat itself. 

It is possible in man and probable in ruminants that fer- 
mentations may occur in the intestines giving rise to fatty acids 
which are possibly converted into fats by the cells of the villi 
or elsewhere. Certain feeding experiments favor the view that 
carbohydrates may be converted into fat or in some way give 
rise to an increase in this substance ; for it is to be borne in 
mind that fat may arise from a certain diet in various ways 
other than its direct transformation into this substance itself. 

There are certain facts that make it clear that fat can be 
formed from proteids : 1. A cow will produce more butter than 
can be accounted for by the fat in her food alone. 2. A bitch 
which had been fed on meat produced more fat in her milk 
than could have been derived directly from her food, and this, 
when the animal was gaining in weight, which is usually to 
be traced to the addition of fat ; so that the fat of the milk 
was not, in all probability, derived from that of the dog's 
body ; and, as will be seen presently, can be accounted for 
without such a supposition. 3. It has been shown by analysis 
that 472 parts of fat were deposited in the body of a pig for 
every 100 in its food. 

These facts of themselves suffice to show that fat can be 
formed from proteid, or at least that proteid food can of itself 
give rise to a metabolismj resulting in fat formation ; and the 
latter is probably the better way to state the case in th6 present 
condition of knowledge. 

An examination of the percentage composition of proteid 
and urea renders a possible construction of fat from proteid 
conceivable and in harmony with other better known physi- 
ological facts. 

Carbon. Hydrogen. Nitrogen. Oxygen. Sulphur. 

Proteid 53-00 7-30 15-53 23-04 1-13 

Urea 20-00 6-66 46-67 26-67 

It will be seen that, if we assume that the urea discharged 
represents the whole of the nitrogen that passes through the 



body, there would remain for disposal otherwise a large amount 
of carbon, for there is nearly three times as much of this ele- 
ment in proteid as in urea ; so that it is possible, from a chemi- 
cal point of view, to understand the origin of fat from the pro- 
teid food ; but too much importance must not be attached to 
such speculations. 

That fat is a real formation, dependent for its composition 
on the work of living tissues, is clear from the well-known fact 
that the fat of one animal differs from that of another, and that 
it preserves its identity, no matter what the food»may be, or in 
what form fat itself may be provided. Certain constituents of 
the animal's fat may be wholly absent from the fat of its food, 
yet they appear just the same in the fat produced under such 
diet. Even bees can construct their wax from proteid, or use 
unlike substances, as sealing-wax. 

But histological examination of forming adipose tissue itself 
throws much light upon the subject. Fat-cells are those in 
which the protoplasm has been largely replaced by fat. The 
latter is seen to arise in the former as very small globules 

Fig. 3*1.— Mammary gland of human female (after LiegeoisV 1, sinus, or dilatation of one of 
lactiferous ducts ; 3, extremities of the ducts ; 3, lobules of gland ; 4, nipple, retracted in 



which run together more and more till they may wholly re- 
place the original protoplasm. 

The history of the mammary gland is, perhaps, still more 
instructive. In this case, the appearance of the cells during 
lactation and at other periods is entirely different. Fat may 

Fig. 335. — Section of mammary gland (udder and nipple) of cow (after Thanhoffer). Ma, sub- 
stance of gland ; N, nipple ; A^ acini of gland : m. d. milk-ducts ; C, milk-cisterns ; /. 
folds in wide milk-ducts ; S, section of sphincter muscle ; s, external skin ; n. m. d, narrow 
milk-duct in nipple. 

be seen to arise Avithin these cells and he extruded, perhaps in 
the same way as an Amoeba gets rid of the waste of its food. 
So far as the animal is concerned, milk is an excretion in a 



It is, in the nature of the case, impossible to foUo-w with 
the eye the formation and separation of milk-sugar, casein, etc. 

Fig. 336.— I. Acinus from mamma of a bitch when inactive (after Heidenhain). U. During 
secretion of milk, a, b, mills-globules ; c, d, e, colostrum-corpuscles ; /, pale cells. 

But the whole process is plainly the work of the cells, and in 
no mechanical sense a mere deposition of fat, etc., from the 
blood ; and the same view applies to the construction of fat by 
connective (adipose) tissue. 

Fig. 337. 

Fig. 8S7.— Human milk-globules, from a healthy lying-in woman, eight days after delivery 

Fig. 388. — Colostrum, from a healthy lying-in woman, twelve hours after delivery (Funke). 

The colostrum-corpuscles are large and granular ; they gradually disappear from the 


Whether fat, as such, or fatty acid, is dealt with without 
being built up into the protoplasm of the cell, is not known ; 
but, taking all the facts into the account, and considering the 
behavior ef cells generally, it seems most natural to regard 
the construction of fat as a sort of secretion or excretion. To 
suppose that a living cell acts upon material in the blood as a 
workman in a factory on his raw material, or even as a chemist 


does in the laboratory, seems to be too crude a conception of 
vital processes. Until it can be rendered very mucli clearer 

Fig. 339.— Microscopic appearances of — ^I, milk : II, cream : III, butter ; IV, colostrum of 
mare ; V, colostrum of cow (after Thanhoffer). 

than at present, it is not safe to assume that their chemistry is 
our chemistry, or their methods our methods. It may be so ; 
but let us not, in our desire for simple explanations or undue 
haste to get some sort of theory that apparently fits into our 
own knowledge, assume it gratuitously, in the absence of the 
clearest proofs, especially when our failures on this supposi- 
tion are so numerous. 

We may say, then, that fat is not merely selected from the 
blood, bnt formed in the animal tissues ; that fat formation 
may take place when the food consists largely of carbohydrates, 
when it is chiefly proteid, or when proteid and fatty. In other 
words, fat results from the metabolism of certain cells, which 
is facilitated by the consumption of carbohydrate and fatty 
food, but is possible when the food is chiefly nitrogenous. We 
must, however, recognize differences both of the species and 
the individual in this respect, as to the extent to which one 
kind of food or the other most favors fat formation (excre- 
tion). The use of the adipose tissue as a packing to pre- 
vent undue escape of heat is evident ; but more important 


purposes are probably served, as will appear from later consid- 

Pathological. — Corpulence, or excessive fat formation, leading 
to tlie hampering of respiration, the action of the muscles, and, 
to a certain extent, many other functions of the body, does not 
arise usually till after middle life, when the organism has 
seen its best days. It seems to indicate, if we judge by the 
frequency of fatty degeneration after disease, that the proto- 
plasm stops short of its best metabolism, and becomes de- 
graded to a lower rank ; for certainly adipose tissue does not 
occupy a high place in the histological scale. Many persons 
given to excessive fat formation are fond of saccharine and 
amylaceous foods ; but the fact that, under the strictest diet, 
the abnormality can be but moderately controlled, shows that 
the main point is the existence vof the habit of certain cells 
naturally to form fat, which, in disease, becomes exaggerated, 
or is taken up by others that normally have little share in 
such work. Such pathological facts throw a good deal of light 
upon the general nature of fat excretion, as it would be better 
to term it, perhaps, and seem to warrant the view that we have 
presented of the metabolic processes. 

Although the nerves governing the secretion of milk have 
not been traced, there can be no doubt that the nervous system 
controls this gland also. The influence of the emotions on both 
the quantity and quality of the milk in the human subject and 
'in lower animals is well known. There seems to be no doubt 
that milk of an injurious if not absolutely poisonous character 
may be formed under the influence of depressing or unusually 
exciting emotions, as grief, rage, etc. We know less about the 
influence of the nervous system in fat formation elsewhere, 
though it is well enough established that persons grow thin 
under worry as well as excessive mental and physical exertion. 
In the latter case, it is not improbable that the overworked 
muscles may draw, in some way, on the stored fat. At the 
same time, fat formation may be interfered with, and be an ex- 
pression of the unnatural conditions generally that have been 
established. Such cases are too complex to permit of being 
completely unraveled. 

Comparative. — While breeders recognize certain foods as 
tending to fat formation and others to milk production, it is 
interesting to note that their experience shows that race and 
individuality, even on the male side, tell. The same conditions 
being in all respects observed, one breed of cows gives more 


and better milk than another, and the bull is himself able to 
transmit this peculiarity ; for, when crossed with other breeds, 
he improves the milking qualities of the latter. Individual 
differences are also well known. 

The Metabolic Processes concerned in the Formation 
OP Urea, Uric Acid, Hippuric Acid, and Allied 

Creatin is represented by the formula C4H9NSOS, and crea- 
tinin by C4H7N3O — ^that is, the latter may be regarded as the 
firmer dehydrated. Creatinin occurs, as we have seen, in urine, 
and the question arises. Is the creatin of muscle the antecedent 
of the creatinin of urine ? Creatin when injected into the 
blood reappears as creatinin in the urine ; but the latter sub- 
stance is not increased by exercise, though the creatin of the 
muscles is, while, like urea, creatin is augmented by a proteid 
(flesh) diet. It is not clear, then, that the creatin of muscle 
has any definite relation to the creatinin of urine. But crea- 
tin occurs not only in muscle, but in a variety of other tis- 
sues, including the nervous ; in fact, it may be regarded as 
one of the products of proteid metabolism. Putting these 
facts along with the absence of urea itself from muscle and 
many other tissues, there is some probability in the view 
that creatin is one of the antecedents of urea ; possibly it is 
one of the products which the kidneys directly convert into 

There are several facts which point to the liver as being 
the seat of urea formation: 1. Leucin, when taken in large 
quantities, reappears in the urine as urea, or, at all events, is 
followed by an increase in the excretion of urea by the kid- 
neys. 2. In certain diseases of the liver (acute atrophy) urea 
is largely replaced in the urine by leucin and tyrosin. Now, 
since the consumption of much proteid matter is soon fol- 
lowed by an excess of urea in the urine, and since in such 
cases it is' likely that a good deal of leucin and its compan- 
ion, tyrosin, are formed in the digestive tract, which we may 
suppose are carried directly by the portal blood to the liver, 
the conclusion has been drawn from this and the facts just 
mentioned, as well as others, that the liver is a former of 


Urea may be prepared artificially, as represented by tbe fol- 
lowing equations : 

1. CO < ONH = C0N,H4 + H,0. 

Ammonium Urea, 


3. CKNH, + H,0 = C0NsH4. 


3. CN(0NH4) = C0N,H4. 


Leuciii is amido-caproic acid (CH80H8CHsCH9CH(NHs) 

Another amido-acid, glycin — 

Amido-acetic acid, 

wben introduced into the digestive tract, gives rise to an in- 
crease of the urea of the urine. 

It will be seen that ammonia compounds, both in the labora- 
tory and apparently in the body, have a formative relation to 
urea ; but beyond this we can not go very far in furnishing a 
chemical explanation of the formation of urea as a part of a 
series of metabolic processes. Do the kidneys merely pick 
out from the blood and pass on into the urinary tubules the 
already formed urea — i. e., eat, so to speak, and then discharge 
it, Amceba-like — or do they manufacture it from bodies that 
have gone on the way a certain distance toward urea before 
they reach the kidneys ; or, again, do they form urea in some 
such way as the mammary gland constructs fat ? 

If the ureters be tied, the renal arteries ligatured, or the 
kidneys extirpated, urea accumulates in the blood and tissues. 
This might be explained on the supposition that urea formed 
elsewhere was not eliminated; or that some body related to 
urea, and the usual transformations of which are completed 
in the kidneys, under these unwonted circumstances becomes 
urea, either in the tissues in which it arose or elsewhere. 

We can not pronounce with certainty in favor of any one 
or all of these conceivable methods. We may perhaps assume 
that creatin and possibly other allied bodies are antecedents of 
urea ; that the leucin and perhaps the tyrosin of digestion in 
some way give rise to urea ; and that the liver and possibly 
the spleen are organs in which a portion of the urea is formed ; 
that a part of the urea of urine is simply withdrawn from the 
blood by the kidneys ; but, as to whether any part is made by 


th.e latter in either of tlie senses to which we have alluded 
ahove, is a matter on which there is very little evidence. It is 
perhaps best to assume, at least, the possibility of the truth of 
both of them. 

Uric Acid. — This substance can be oxidized in the laboratory 
to urea, thus : 

aH4N403 + H,0 + = CaN-^H^Oi + CNsH^O, 

Uric acid. Alloxan. Urea. 

SO that it has been assumed that uric acid in the body is a stage 
short of urea, and this seemed the more plausible, since it re- 
places the latter in the cold-blooded animals. But this is not 
entirely the case, for in the frog urea is found in the urine, 
and our knowledge of this secretion in most of them is very 
incomplete ; moreover, in the birds, representing the very great- 
est degree of activity and the highest oxidative capacity, uric 
acid is the principal nitrogenous body of the urine, and not 

Pathological. — When there is excessive indulgence by man 
in proteid foods, etc., the uric acid, normally small in quantity, 
is increased greatly, and may give rise to depositions of urates 
about the joints. 

It seems best to regard uric acid as the result of proteid 
metabolism when of a certain type, and urea as the outcome 
of the vital processes of animals of a distinct physiological 

Evolution. — There is a good deal of paleontological evidence 
which points to a phylogenetic (ancestral) relation between 
birds and reptiles ; hence the many points of functional resem- 
blance between these groups of creatures now so different in 
form and, in some respects, in functions. The excessive pro- 
duction of uric acid (uric-acid diathesis) can be understood in 
the light of physiological reversion. It is well known that this 
diathesis is hereditary — that is to say, the metabolic habit of 
excessive production of uric acid may be imparted to offspring. 

Hippuric Acid. — Among the herbivora hippuric acid may be 
said to replace uric acid. In the laboratory this acid may be 
made from benzoic acid and glycocol (glycin), thus : 

CeHa.OOOH + HaC>^^^ = CH,<^^^g^'^'^' + H,0. 

Benzoic acid. Glycin. Hippuric acid. 

It is interesting to note that, when benzoic acid is swallowed 
by man, hippuric acid appears in the urine ; and it is said that 


wlieii blood containing benzoic acid is mixed with f resb minced 
kidney it is transformed to hippuric acid. Hay contains a ben- 
zoic compound, so that it is not difficult to find a starting-point 
for the hippuric acid of the herbivora. In these instances it is 
assumed that glycin is added in the kidneys ; but, as a matter 
of fact, this substance has not as yet been found anywhere in 
the body, though it is possible to conceive that, like peptone, 
it might be formed and disappear (be used) as fast as gen- 

The above is one of the clearest cases favoring the view that 
the chemical processes of the body do really very much resem- 
ble those of the laboratory. But, considering the difficulty as 
to glycin, and that the liver also can form hippuric acid under 
similar circumstances (those mentioned above), and that there 
are several laboratory methods for the synthesis of hippuric 
acid, it behooves us to be cautious even in this case, the chain 
■ of facts being by no means complete. 

Of the origin of the allied bodies — xanthin, etc. — or their 
fate and purpose, we know very little. Their resemblance 
chemically to certain alkaloids in tea, coffee, etc., is suggestive. 
Are they natural stimulants ? 

The Study of the Metabolic Processes by other 

It will be abundantly evident that our attempts to follow 
the changes which the food undergoes from the time of its 
introduction into the blood until it is removed in altered form 
from the body has not been as yet attended with great success. 
It is possible to establish relations between the ingesta and the 
egesta, or the income and output which have a certain value. 
It is important, however, to remember that, when quantitive 
estimations have to be made, a small error in the data becomes 
a large error in the final estimate; one untrue assumption 
may vitiate completely all the conclusions. 

In discussing the subject we shall introduce a number of 
tables, but it will be remembered that the results obtained by 
one investigator differ from those obtained by another ; and 
that in all of them there are some deviations from strict ac- 
curacy, so that the results must be regarded as only approxi- 
mately correct. It is, however, we think, better to examine 
such statistical tables of analyses, etc., than to rely on the 
mere verbal statement of certain results, as it leaves more 




room for individual judgment and the assimilation of such 
ideas as they may suggest outside of the subject in hand. 

The subject of diet is a very large one ; but it will he evi- 
dent on reflection that, before an average diet can be prescribed 
on any scientific grounds, the composition of the body and 
the nature of those processes on which nutrition generally 
depends must be known. Not a little may be learned by an 
examination of the behavior of the body in the absence of all 
diet, when it may be said to feed on itself, one tissue sup- 
plying another. All starving animals are in the nature of the 
case carnivorous. 

Composition of the Mammalian Body. 

Adult man. 

New-born child. 






Thoracic viscera 


Abdominal viscera 





For the cat an analysis has yielded the following : 

Muscle and tendons 45"0 per cent. 

Bones 14-7 

Skin , . 13-X) " 

Mesentery and adipose tissue 3"8 " 

Liver 4-8 " 

Blood (escaping at death) 6'0 " 

Other organs and tissues 1.3'7 " 

The large proportional weight of the muscles, the similarly 
large amount of blood they receive; which is striking in the 
case of the liver, also suggest that the metabolism of these 
structures is very active, and we should expect that they 
would lose greatly during a starvation period. It is a matter 
of common observation that animals do lose weight and grow 
thin under such circumstances, which means that they must 
lose in the muscles and the adipose tissue. Attempts have been 
made to determine exactly the extent to which the various 
tissues do suffer during complete abstinence from food, and 
this may be gathered from the table given below. 

Starvation. — A cat weighing 2,464 grammes lost before death 
on the eighteenth day 1,197 grammes in weight. Of this about 


204 grammes (17 per cent) was in albuminous matter; 132 
grammes (11 per cent) loss of fat ; 863 grammes loss of water, 71 
per cent of the total body weight. 

It will not be forgotten that about three fourths of the 
body is made up of water, so that the loss of so large an 
amount of the latter during starvation is not wholly inexpli- 

In the case of another cat during a starvation period of thir- 
teen days 734 grammes of solids were lost, of which 248 grammes 
were fat and 118 muscle — i. e., about one half of the total loss 
was referable to these two tissues alone. 

The other tissues lost as follows, estimated as dry solids : 

Adipose tissue 97'0 per cent. 

Spleen 631 

Liver 56-6 " 

Muscles 30'2 " 

Blood 17-6 

Brain and spinal cord O'O " 

It will be observed (a) that the loss of the fatty tissue was 
greatest, nearly all disappearing ; (fe) that the glandular struct- 
ures were next in order the greatest sufferers; (c) that after 
them come the skeletal muscles. 

Now, it has been already seen that these tissues all engage 
in an active metabolism with the exception of adipose tis- 

The small. loss on the part of the heart, which is still less 
for the nervous system, is especially noteworthy. Two ex- 
planations are possible. On the one hand, we may suppose 
that their metabolism is active, but that they feed in some 
sense on the other tissues, and thus preserve themselves from 
loss of substance. But, again, we have seen that the functional 
activity of the nervous system is not accompanied by any very 
marked chemical phenomena that we have succeeded in detect- 
ing, at all events ; and little is known of the metabolism of 
the heart itself. Do its pulsations from long habit go on with 
little expenditure of energy, as is the case with the automatic 
workman engaged in a narrow round of duty ? Has the nerv- 
ous system in the course of its evolution acquired the power 
of accomplishing much, like persons with special -aptitudes, 
with little loss of energy ? It is not possible to decide exactly 
what share these several factors may take ; though that they 
all and others as yet unrecognized do share in the general 
result seems probable. The loss of adipose tissue is so striking 


that we must regard it as an especially valuable storehouse of 
energy, available as required. 

When we turn to the urine for information, it is found that 
in the above case 37 grammes of nitrogen were excreted and 
almost entirely, of course, in the form of urea ; and since the 
loss of nitrogen from the muscles amounted to 15 grammes, it 
will appear that more than one half of the nitrogenous excreta 
is traceable to the metabolism of muscular tissue. It has been 
customary to account for the urea in two ways : first, as derived 
from the metabolism of the tissues as such, and continuously 
throughout the whole starvation period ; and, secondly, from a 
stored surplus of proteid which was assumed to be used up 
rapidly during the early days of the fasting, and was the luxus 
consumption of certain investigators. 

Comparative. — Experiment has shown that the length of 
time during which different groups of animals can endure com- 
plete withdrawal of food is very variable, and this applies to 
individuals as well as species. That such differences hold for 
the human subject is well illustrated by the history of the sur- 
vivors of wrecks. Making great allowances for such devia- 
tions from any such results as can be established by a limited 
number of experiments, it may be stated that the human being 
succumbs in from twenty-one to twenty-four days ; dogs in 
good condition at the outset in from twenty-eight to thirty 
days; small mammals and birds in nine days, and frogs in 
nine months. Very much depends on whether water is allowed 
or not — ^life lasting much longer in the former case. The very 
young and the very old yield sooner than persons of middle 
age. It has been estimated that strong adults die when they 
lose -^ of the body weight. "Well-fed animals lose weight 
more rapidly at first than afterward. 

Diet. — All experiments and observations tend to show that 
an animal can not remain in health for any considerable period 
without having in its food proteids, fats, carbohydrates, and 
salts ; indeed, sooner or later deprivation of any one. of these 
will result in death. 

Estimates based on many observations have been made of 
the proportion in which these substances should enter into a 
normal diet. In the nature of the case, for a creature like 
man especially, whose adaptive power is so great that he can 
learn to live under a greater variety of conditions than any 
other animal, any figures on this subject must be interpreted 
as being but a very general statement of the case. 



We give another series of tables, founded on experiments 
by different investigators from whicli a number of conclusions 
may be drawn : 

The Requirements of an Adult Man for Twenty four Hours. 


At rest. 

Moderate work. 

Laborious work. 

(Playfair.) ■ 

(V. Pettenkofer 
and V. Voil.) 














Ingesta of an Adult working moderately (Vierordt): 




120 ffraniTiies albumin, containing 







90 fframmcs fats, containing 


330 grammes starch, containing 







It has further been estimated that 744 grammes of oxygen 
are respired, 2,818 grammes water drunk, and 32 grammes of 
salts consumed. 

The total ingesta have been estimated at -^ of the body 
weight ; and the daily metabolism of the body is calculated as 
leading to the transformation of 6 per cent of the water, 6 per 
cent of the fat, 1 per cent of the proteids, and 4 per cent of 
the salts of the body. 

The Egesta of an 

Adult working moderately. 





By respiration 









3-3 ' 





By fffices 








If we lay down the rule as has been done, that the nitrog- 
enous should bear the proportion of 1 to 3|-4|- of non-ni- 
trogenous, an inspection of the following analytical table 
will show how these various food-stuffs conform to such an 



For the herbivora from 1 to 8-9 (some claim 1 to 5J) is the 
estimated ratio of nitrogenous to non-nitrogenous foods : 

Nitro. Non-nitro. 

Human milk 10 37 

Wheaten-flour 10 46 

Oatmeal 10 50 

Rye-meal 10 57 

Barley-meal 10 57 

White potatoes 10 80 

Blue potatoes. 10 115 

Rice 10 123 

Buckwheat- meal 10 130 






Hare's flesh 





















Cow's milk 



One investigator estimates that in order to get the one hun- 
dred and thirty grammes of proteids required by an adult man 
engaged at moderate labor, the following proportions of differ- 
ent kinds of foods must be eaten : 


Cheese 388 

Lentils 491 

Peas 582 

Beef 614 

Eggs 968 


Wheaten bread 1,444 

Rice 2,562 

Rye-bread 2,875 

Potatoes 10,000 

One conclusion that is most obvious from the above is that, 
in order to obtain the amount of proteids nee'ded from certain 
kinds of food, enormous quantities must be eaten and digested ; 
and as there would be in such cases an excess of carbohydrates, 
fats, etc., unnecessary work is entailed upon the organism in 
order to dispose of this. 

Feeding Experiments {Ingesta and Egesta). 

If all that enters the body in any form be known, arid all 
that leaves it be equally well known, conclusions may be drawn 
in regard to the metabolism the food has undergone. The pos- 
sible sources of fallacy will appear as we proceed. 

The ingesta, in the widest sense, include the respired air as 
well as the food ; though from the latter must be subtracted 
the waste (undigested) matters that appear in the faeces. The 
ingesta when analyzed include carbon, hydrogen, oxygen, ni- 
trogen, sulphur, phosphorus, water, and salts, their source 
being the atmosphere and the food-stuffs. 

The egesta the same, and chiefly in the form of carbonic an- 
hydride, of water from the lungs, skin, alimentary canal, and 


kidneys, of salts and water from the skin and kidneys, and of 
nitrogen, cMefly as urea almost wholly from the kidneys. Usu- 
ally in experimental determinations the total quantity of the 
nitrogen of the urine is estimated. If free nitrogen plays any 
part in the metabolic processes it is unknown. 

A large number of feeding experiments have been made by 
different investigators, chiefly, though not exclusively, on the 
lower animals. Some such method as the following has usu- 
ally been pursued : 1. The food used is carefully weighed and a 
sample of it analyzed, so that more exact data may be obtained. 
3. The amount of oxygen used and carbonic anhydride exhaled, 
as well as the amount of water given off in any form, is esti- 
mated. 3. The amount of the nitrogenous excreta is calculated, 
chiefly from an analysis of the urine, though any loss by hair, 
etc., is also to be taken into account. 

It has been generally assumed that the nitrogen of the ex- 
creta represents practically the whole of that element entering 
the body. This has been denied by some investigators. 

The respiratory products have been estimated in various 
ways. One consists in measuring the quantity of oxygen sup- 
plied to the chamber in which the animal under observation is 
inclosed, and analyzing from time to time samples of the air as 
it is drawn through the chamber ; and on these results the total 
estimates are based. 

It will appear that even errors in calculating the composi- 
tion of the food — and this is very variable in different samples, 
e. g., of flesh ; or any errors in the analysis of the urine, or in 
the more difficult task of estimating the respiratory products, 
may, when multiplying to get the totals, amount to serious de- 
partures from accuracy in the end ; so that all conclusions in 
such a complicated case must be drawn with the greatest cau- 
tion. But it can not be doubted that such investigations have 
proved of much practical and some scientific value. The labor 
they entail is enormous. 

Proteid Metabolism. — If we conceive of a structural unit or 
cell as made up of a genuine protoplasm constituting its mesh- 
work and holding in the interstices certain substances that are 
not part of itself, strictly speaking, the question ai'ises, Are 
these latter used up in the metabolic process as such, or do they 
become a part of the true protoplasm before they undergo the 
changes referred to above ? Some writers speak of " organ 
albumin " and " circulating albumin," and they believe that the 
latter, by which is meant the proteid material found every- 


where in the fluids of the hody, as opposed to the former as 
constituting organized tissues, undergoes changes of a retro- 
grade kind without ever becoming organ albumin, while the 
term luxus consumption was applied to the metabolism of pro- 
teids in the blood. The latter is not now believed to occur. 
But whether a portion of the urea that represents, in the main, 
the results of proteid metabolism is not derived from the 
metabolism of the material in the interspaces of the tissues 
(circulating proteids on which the cells are supposed to act 
and in which they effect changes without making these pro- 
teids a part of themselves), is uncertain. 

Nitrogenous Ec[uilibrium. — It is possible to so feed an animal, 
say a dog, that the total nitrogen of the ingesta and egesta 
shall be equal ; and this may be accomplished without the ani- 
mal losing or gaining weight appreciably or again while he is 
gaining. If there be a gain, it can usually be traced to the 
formation of fat, so that the proteid, we may suppose, has 
been split up into a part that is constructed into fat and a 
part which is represented by the urea, the fat being either used 
up or stored in the body. Moreover, an analysis of a pig that 
had been fed on a fixed diet, and a comparison made with one 
of the same litter killed at the commencement of the experi- 
ment; showed that of the dry nitrogenous food only about 
seven per cent in this animal and four per cent in the sheep 
had been laid away as 'dry proteid. It is perfectly plain, then, 
that proteid diet does not involve only proteid construction 
within the body. 

Comparative. — The amount of flesh which a dog, being a car- 
nivorous animal, can digest and use for the maintenance of his 
metabolic processes is enormous ; though it has been learned 
that ill-nourished dogs can not even at the outset of a feeding 
experiment of this kind maintain the equilibrium of their body 
weight on a purely flesh diet (fat being excluded). They at 
once commence to lose weight — i. e., they draw upon their own 
limited store of fat. 

The digestion of herbivora being essentially adapted, to a 
vegetable diet, they can not live at all upon flesh, while a dog 
can consume for a time without manifest harm ^ to ^ of its 
body- weight of this food. 

Man, when fed exclusively on meat soon shows failure, he 
being unable to digest enough to supply the needed carbohy- 
drates, etc. But the large amount ' of urea in the urine of car- 
nivorous animals generally, and the excess found in the urine 


of man when feeding largely on a flesh diet, show that the pro- 
teid metabolism is under such circumstances very active. 

It is also a well-known observation that carnivorous ani- 
mals (dogs) are more active and display to a greater extent 
their latent ferocity, evidence of their descent from wild car- 
nivorous progenitors, when like them they feed very largely on 
flesh! The evidence seems to point pretty clearly to the con- 
clusion that a nitrogenous (flesh) diet increases the activity of 
the vital processes of the body, and especially the proteid me- 

Some have explained this result on the assumption that 
such diet led to an increase in the red corpuscles of the blood, 
and hence in the oxygen-supply ; but mere abundance of sup- 
ply will never of itself explain results in a living organism. It 
may be and probably is true that such a diet augments the 
activity of the oxidative processes, but the reason of this lies 
deeper, we think, than the explanations as yet offered assume. 
That an excess of proteids may be stored, as it seems, is true of 
fats and carbohydrates, to be used in the hour of need, seems 
not improbable, though this has not as yet been shown to be 
the case. But in all these considerations it must be borne in 
mind that the metabolic processes go on in the tissues and not 
in the blood, and probably not in the lymph. Not that these 
fluids (tissues) are without their own metabolic processes for 
and by themselves ; but what is meant to be conveyed is that 
the metabolic processes of the body generally do not take place 
in the blood. 

The Effects of Gelatine in the Diet. — Actual experiment shows 
that this substance can not take the place of proteid, though it 
also makes it evident that less of the latter sufiices when mixed 
with a certain proportion of gelatine ; and it has been suggested 
that it is split up into a fatty portion and urea, and that it thus, 
by aiding in the formation of fat, preserves some of the proteid 
for other uses than fat construction. This theory, however, is 
not well substantiated. It will be borne in mind that ordinary 
flesh contains, as we find it naturally in the carcass, not only 
some fat, but a good deal of fibrous tissue, which can be con- 
verted by heating into gelatine. 

Fats and Carbohydrates. — It is a matter of common observa- 
tion and of more exact experiment that even a carnivorous ani- 
mal thrives better on a diet of fat and lean meat than on lean 
flesh alone. Thus, it has been found that nitrogenous equi- 
libi'ium was as readily established by a due mixture of fat and 


lean as upon twice the quantity of lean flesh alone. It is plain, 
tlien, that the metabolism is actually slowed by a fatty diet. 
When an animal is given but little fat, none whatever is laid 
up, but all the carbon of the fat can be accounted for in the 
excreta, chiefly as carbonic anhydride. Again, the fatty por- 
tion remaining constant, it has been found that increasing the 
proteid leads not to a storage of thejcarbon of the proteid ex- 
cess, but to an increased consumption of this element. It is 
then possible to understand how excessive consumption of pro- 
teids may lead, as seems to be the case, to the disappearance of 
fat and loss of weight, so that a proteid diet increases not only 
nitrogenous but non-nitrogenous metabolism. That carbohy- 
drates mixed with a due proportion of the other constituents 
of a diet do increase fat formation is well established ; though 
there is no equally well-grounded explanation of how this is 
accomplished. Upon the whole, it seems most likely that fat 
can be directly formed from carbohydrates, or, at all events, 
that they directly give rise to fat if they are not converted 
themselves into that substance. 

Comparative. — It is found that there are relations between 
the food used and the quantity of carbonic dioxide expelled 
which are instructive. The formula following show the amount 
of oxygen necessary to convert a starch and a fat into carbonic 
anhydride and water : 

1. CsHioOs + 0,, = 6(CO0 + 5(H,0). 

2. a,H.«Oe + 0,60 = 57(CO,) + 62{H,0). 

It will be observed that in the first case the oxygen used to 
oxidize the starch has all reappeared as CO2, while in the sec- 
ond only 114 parts out of 160 so reappear. As a matter of fact, 
more of the oxygen used does in herbivora reappear as COs, 
and less as water, while the reverse holds for the carnivora., the 
proportion being, it is estimated, as from 90 to 60 per cent. 
This is to be explained by the character of the food in each 
instance, for this relation no longer holds during fasting, when 
the herbivorous animal becomes carnivorous in the sense that 
it consumes its own tissues. 

To most persons the carbohydrates are more digestible than 
fats, though they have less potential energy, as will shortly 
be seen. 

The Effects of Salts, Water, etc., in the Diet. — We have already 
considered how salts in the form of condiments may beneficially 
influence the digestion ; but, when we come to inquire as to the 


part they play when introduced into the blood, we soon find 
that our knowledge is very limited. 

Sulphur, and especially phosphorus, seem to have some im- 
portant use which quite eludes detection. It is important to 
remember that certain salts are combined with proteids in the 
body, possibly to a greater extent than we can learn from the 
mere analysis of dead tissues. 

Pathological. — The withdrawal of any of the important salts 
of the body soon leads to disease, clear evidence in itself of their 
great importance. This is notably the case in scurvy, in which 
disease the blood seems to be so disordered and the nutrition 
of the vessel-walls so altered that the former (even some of the 
blood-cells) passes through the latter. 

Water. — The use of water certainly has a great influence 
over the metabolic processes of the body. The temporary ad- 
dition or withdrawal of even a few ounces of water from the 
regular supply of a dog in the course of a feeding experiment 
greatly modifies the results obtained for the time. . It is well 
known that increase of water in the diet leads to a correspond- 
ing increase in the amount of urea excreted. It is likely that 
even yet we fail to appreciate the great part which water plays 
in the animal economy. 

The Energy of the Animal Body. 

As already explained, we distinguish between potential or 
latent and actual energy. All the energy of the body is to be 
traced to the influence of the tissues upon the food. Energy 
may be estimated as mechanical work or as heat, and the 
one may be converted into the other. All the processes of 
the organism involve chemical changes, and a large propor- 
tion of these are of the nature of oxidations ; so that, speak- 
ing broadly, the oxidations of the animal body are the sources 
of its energy ; and in estimating the quantity of energy, either 
as heat or work, that a given food-stuff will produce, one must 
consider whether the oxidative processes are complete or par- 
tial • thus, in the case of proteid food, if we suppose that the 
urea excreted represents the form in which the oxidative pro- 
cesses end or are arrested, we must, in estimating the actual 
energy of the proteid, subtract the amount of energy that 
would be produced were the urea itself completely oxidized 

If the amount of heat that a body will produce in its com- 



bustion beknown, then by the law of the conversion and equiv- 
alence of energy the mechanical equivalent can be estimated in 
that particular case. 

The heat-producing power of different substances can be 
directly learned by ascertaining the extent to which, when fully 
burned (to water and carbonic anhydride), they elevate the 
temperature of a given volume of water ; and this can at once 
be translated into its mechanical equivalent of work, so that 
we may say that one gramme of dry proteid would give rise to 
a certain number of gramme-degrees of heat or kilogramme- 
metres of work. A few figures will now show the relative 
values of certain food-stuffs : 

1 gramme proteid 

1 gi'amme urea 

Available energy of the proteid 







The reason of the subtraction has been explained above. 

Taking another diet in regard to which the estimates differ 
somewhat from those given previously, but convenient now as 
showing how equal weights of substances produce very dif- 
ferent amounts of energy, we find that — 

100 grammes proteid yield 
100 grammes fat yield", . . . 
240 grammes starch yield. 








In other words, nearly a million kilogramme-metres of en- 
ergy are available from the above diet for one day, provided 
it be all oxidized in the body. 

Food-stuffs, then, with the oxygen of the air, are the body's 
sources of energy. What are the forms in which its expendi- 
ture appears ? We may answer at once, heat and mechanical 
work ; for it is assumed that internal movements, as those of 
the viscera, and all the friction of the body, all its molecular 
motion, all secretive processes, are to be regarded as finally 
augmenting the heat of the body. Heat is lost by the skin, 
lungs, urine, and faeces. 


The amount of work whicli a man or other animal can do 
on a given diet may be estimated without the same sources of 
fallacy as attend the calculation of the heat expenditure ; for, 
"when an animal is confined in a calometric chamber, the con- 
ditions of the norrnal metabolism are not observed. 

The SorrRCES of Muscular Energy. 

Experimental. — Two physiologists (Fick and Wislicenus) as- 
cended a mountain, noting the conditions under which their 
metabolism was performed, and drew certain conclusions in re- 
gard to the question now being considered. They lived exclu- 
sively on a non-nitrogenous diet while the work was being done, 
and estimated the amount of urea excreted at the same time. 
Assuming that the urea does represent the proteid metabolism 
(oxidation) which bore, of course, a definite relation to the 
energy available, it was found that in the case of each of them 
this was only about half enough to account for the work done. 
Even making large allowances for error in the estimates, if 
this experiment is to be trusted at all, it is plain that the 
energy of the muscles of the body is not derivable from their 
proteid metabolism ; and there are other facts which point in 
the same direction. 

It is found, when an isolated muscle is studied, that its 
continued contraction does not produce nitrogenous bodies, but 
very different ones, such as carbonic anhydride. The quantity 
of the latter may be augmented many times by work. But it 
is no longer believed that the severest labor appreciably in- 
creases the secretion of urea. 

The division of foods into heat-producers and tissue-btiilders 
is unjustifiable, as will appear from what has just been stated, 
as well as from such facts as the production of fat from proteid 
food, thus showing that the latter is indirectly a producer of 
carbonic anhydride, assuming that fat is oxidized into that 

Animal Heat. 

Though a large part of the heat generated within the body 
is traceable to oxidations taking place in the tissues, it is better 
to speak of the heat as being the outcome of all the chemical 
processes of the organism ; and though heat may be rendered 
latent in certain organs for a time, in the end it must reappear. 
While all the tissues are heat-producers (thermogenic), the ex- 


tent to wMch they are such -woiild depend, we should suppose, 
upon the degree to -which they were the seat of metabolic pro- 
cesses ; and actual tests establish this fact. Thus, among glands 
the liver is the greatest heat-producer ; hence the blood from 
this organ is the warmest of the whole body. The muscles also 
are especially the thermogenic tissue. 

The temperature of the blood in the hepatic vein is warmer 
than that in the portal, a clear evidence that the metabolism of 
this organ has elevated the temperature of the blood flowing 
through it. 

The temperature of the blood (its own metabolism being 
slight) is a pretty fair indication of the resultant effect of the 
production and the loss of heat. 

For obvious reasons, the temperature of different parts of 
the body of man and other animals varies. 

The statements of observers in regard to the temperature of 
various animals and of different parts of the body disagree in a 
way that would be puzzling, were it not known how difficult it 
is to procure perfectly accurate thermometers, not to mention 
individual differences. The axillary temperature is about 
37'5° C. ; that of the mouth a little higher, and of the rectum or 
vagina slightly more elevated. The mean temperature of the 
blood is placed at 39° C. 

It is a very striking fact, however, that the different parts 
of the body ordinarily accessible by a thermometer vary so 
little — not more perhaps than a degree or a degree and a half. 
The temperature of the hepatic vein has been put down as 397°, 
and it contains the warmest blood of the body. 

Comparative. — The temperature of various groups of animals 
has been stated to be as follows : Gull, 37"8° ; swallow, 44"03° ; 
dolphin, 35"5°; mouse, 41"1°; snakes, 10° to 13°, but higher in large 
specimens (python). Cold-blooded animals have a temperature 
i a little higher (less than 1° C. usually) than the surrounding air. 
; During the swarming of bees the hive temperature may rise 
from 32° to 40°. All cold-blooded animals have probably a 
higher temperature in the breeding-season. In our domestic 
mammals the normal temperature is not widely different from 
that of man. 

Variations in the average temperature are dependent on 
numerous causes which may affect either the heat produc- 
tion or heat loss : 1. Change of climate has a very slight but 
real influence, the temperature being elevated a fraction of 
a degree when an individual travels from the poles toward 



tlie equator, and the same may be said of tlie effect of tlie ' 
temperature of a warm summer day as com^pared with, a cold 
winter one. The wonder is that, considering the external 
temperature, the variation is s